The vegetation of the Kozara National Park is the result of a mutual adjustment between biodiversity and habitat conditions in a very long period of the development of the living world, with significant impact of anthropogenic activities. From the standpoint of horizontal specification of vegetation, the entire Kozara massif belongs to the lower forest belt of Illyrian hornbeam area- Carpinion betuli illyrico-podolicum.
From the standpoint of the vertical specification, the gentler slopes in the lower (submountainous) zone are commonly covered in sessile oak and plain hornbeam forest- Querco-Carpinetum croaticum. The higher (mountainous) zones, and colder locations of submountainous belt, are dominated by beech and fir forests -Abieti-Fagetum praepannonicum. These forests are widespread in most parts of the Park. They absolutely dominate the central and northern, colder, part of the National Park. The fir is the dominant species, which can be found at very low latitudes (up to 200 m) on the north side of Kozara Mountain.
The sessile oak forests -Festucodrymeiae Quercetum petraeae spread on exposed slopes and steep slopes of the south-western part of the National Park.
Part of the southern Mrakovica is covered in fir and oak forests (Quercum-Abietetum), which is in fact a transition between vegetation communities of Abieti-Fagetum praepannonicum and Festuco drymeiae Quercetum petraeae which represents a specific feature of the Kozara National Park.
Beech forests in the belt of oak and hornbeam forests belong to the vegetation community Fagetum submontanum. The largest areas are in the south-western and southern parts of the region.
Sessile oak and hornbeam Querco Carpinetum forests can be found in the lower parts of the slopes, on the gentler slopes and plateaus.
Black hornbeam, pubescent oak, elm and black ash tree forests cover higher and steeper parts of the limestone slopes, where the soil consists of shallow layer of fertile humus.
88% of the territory of the Kozara National Park is covered in forests. Forests in this area have generally protective function which influences the survival and preservation of plant communities, genetic resources and species in a more natural state.
The most common forests in the Kozara National Park are high beech and fir forests. The dense complexes of these forests are especially beautiful in the autumn when the autumn colours and nuances, like red, yellow and brown mix with the green colour of coniferous trees.