The relief image of the protected area is roughly determined by anticline ridges and syncline valleys between them. The vibrant activity of endogen and exogenous forces significantly changed the basically simple structure. There are three natural-geographic units according to relief properties: hills, hillocks and valleys. The eastern part of the Posavje Hills is placed into the first unit. The second unit (hillocks) consists of the area between the Sotla and the Bistrica and the area south of the Orlica. There are two major valleys; the Bistrica and the Sotla valley. They run almost parallel until the Bistrica turns east at Trebče.

The surface is divided by valleys, gorges and ravines, owing to diverse geological structure with non-permeable erodible rocks. There are frequent relief inversions. Non-permeable rocks are the reason for the dense river shed. Rivers and streams cut deep valleys with both lateral and down cutting erosion. On the slopes, the weathering suddenly slips and is rinsed away at the same time owing to the different slope and parent material. The outstanding relief is from lithothamnium limestone, resistant to erosion (interesting examples of selective erosion).

Tertiary hills are marked by the above-average presence of lithothamnium limestones that occur mostly in central part. Marl, sand and sandstone prevail on the Sotla's side of the hills. In the lithothamnium limestones belt between Drensko rebro, Zdole, Križan Vrh and Podsreda the surface is karstic, with its typical funnel-shaped holes, caves, abysses and dry valleys.  In the Tertiary marl and sandstone belt, there are frequent landslides that occur during the heavy rain periods.  The middle part of Sotla hills does not have one central spacious valley but two transversal valleys of the Sotla and the Bistrica.

Here, tectonic rising started later and it hasn't stopped yet. However, it was less intense as in the neighbour Alps. The area is considered prone to earthquakes. In 1974, the area suffered a so-called Kozjansko earthquake with the epicentre in the Zibiški potok valley.

The valley of Bistrica runs along the western part of the area. It widens between Pilštanj and Kozje. The bottom width of the valley changes all the time; it is wider on the flood plains (Lesično, Kozje, Podsreda) and narrower when it passes the resistant rocks of the eastern Posavje Hills.  It even becomes a gorge when it cuts through the thick layers of dolomite near Kozje. In Podsreda, the river reaches the layers of poorly-resistant Tertiary rocks and it makes a sudden turn to east. Between the Orlica and the Bukovica, near Trebče, there is a 3 km epigenetic gorge.

A wide plain between the Bistrica and the Sotla opens at Zagaj and continues to Kumrovec plain in Croatia.

Hills and hillocks were cut from NW to SE by the Sotla that cuts through the east Orlica below Kunšperk (and after Zelenjak gorge) and then turns south. Flood plains along the Sotla are much wider than the ones along the Bistrica, owing to the less resistant Tertiary sediments. Hills, hillocks and plains belong to the fluvial or fluvial-Karst relief type. Surface and subterranean Karst phenomena are located within the so-called »isolated Karst area« that belongs both to Tertiary and Mesozoic.

Various forms of erosion such as accumulation, weathering, denudation and karstification formed the followin geomorphological phenomena in Kozjanski Park:

	Ajdovska žena (Ajdovska woman) – Pilštanj Ajdovska žena (Ajdovska woman) is a picturesque rock created by erosion of grained dolomite on the steep forested slope above the left bank of Bistrica (Pilštanj). The antropomorph rock formation is 12 metres tall (7 metres on the gradual side) and up to 2 metres wide.  It is in a shape of a woman carrying a child in her arms. According to the legend this woman turned to stone after she had cursed the sun.
	The Bistrice gorge The Bistrica river cut a 3 km gorge between Trebče and Zagaj. The gorge becomes extremely narrow and impassable between Rebrija and Tisovec. The banks are steep, rocky, forested and rise over 400 metres on the southern side. This is the most picturesque and preserved gorge in the eastern Slovenia and it provides an example of a well-protected river ecosystem.
	Zelenjak gorge The Sotla cut through the eastern slope of the Oslica and created a 1.5 km dolomite Zelenjak gorge that opens in Cesarsko brdo in Croatia. The gorge located below Kunšperk is surrounded by steep, forested and sometimes even rocky slopes. Tectonic rising in the Posavje Hills didn't start until the end of Tertiary period. Thus, the Sotla cut through the denuded Triassic anticline. Its gorge is a geomorphologic phenomenon of national importance and it will become a subject of cross-border cooperation between Slovenia and Croatia. .
	Gruška cave (Hermit's cave) The cave, which is located at the fault in the stratified Miocene limestone, displays the typical shape of a dry cave passage that has been strongly eroded. The cave used to be a spring but erosion and corrosion later formed a passage. Forms of secondary karstification are already appearing at the end of the passage (stalactites, sinter formations). The total length of the cave is 22 m, and the altitude difference is 2 m. The cave was given its name after the nearby source of the Gruška. Locals call the cave the Hermit's Cave, because a hermit lived in it during and after the First World War. According to the legend, the cave used to be a wealthy square that sank after his inhabitants had held the forbidden festivities on Good Friday.
	Dry walley with a sinkhole It is located in Gorjane above Podsreda. Surface water disappears underground through the funnel-shaped sinkhole.
	Vilna peč in Bredič above Lesično There is a cave under the precipitous rock approximately 60 m under the Bredič ridge. It was formed in dolomite, along the vein of dolomite breccia.  The breccia layer was removed because of weathering and gravitation and left an 8 m wide and 4 m long cave with two entrances.
	Pustišek cave The cave is located on the margin of a large sinkhole in Lastič. The entrance is an inclined sinking abyss. After 8 m the entrance tunnel widens into a hall, followed by another one consisting of an upper and a lower tunnel. The cave is of typical erosion-corrosion origin. There is an active stream that still erodes and transforms the cave.