These research was carried out in the northeastern part of the NP Risnjak along the river Kupa. The flora and vegetation of the common beech forests (Fagus sylvatica L.) were studied. Braun-Blanquet's research method was used, and 19 releves was recorded. Total number of 162 plant species were recorded in four different associations. These are Blechno-Fagetum, Lamio orvalae-Fagetum, Helleboro nigri-Fagetum and Ostryo-Fagetum. The Blechno-Fagetum association grows on the western slopes along the river Kupa, on distric brown soils above the silicate substrate. Its differential species are Blechnum spicant, Luzula luzuloides, L. pilosa, Vaccinium myrtillus and other acidophytes. Beech forest with dead nettle (Lamio orvalae-Fagetum) spreads on the eastern slopes of the river Kupa, and is characterized by the abundance of species of the Illyrian floral geoelement. Among them, Lamium orvala, Omphalodes verna, Calamintha grandiflora and Cardamine enneaphyllos are standing out. Association Helleboro-Fagetum grows in a larger complex, on a dry, shallow rendzine on the dolomite supstrate. The distinguished differential species are Carex alba, C. flacca, Helleborus niger and Buphthalmum salicifolium. The fourth association Ostryo-Fagetum is present in fragments, on extremely steep, exposed and rocky localities. In the floral composition the most significant are thermophilic species, and differential species are Ostrya carpinifolia, Cornus mas, Melittis melissophyllum, Vincetoxicum hirundinaria and others. This research has complemented the knowledge of the beech forests of the Risnjak National Park and the wider Dinaric area.