The northern edge of the distribution area of European hornbeam (Carpinus betulus L.) makes the border between northern and southern geographical belts of northeastern sector of Central European province. On the territory of Lithuania Quercus robur forests with Carpinus betulus (ass. Tilio-Carpinetum) are the climate climax vegetation of southern belt, while Quercus robur forests without Carpinus betulus belong to the northern one. Flora composition of these oakwoods was compared. Gradual changes in the flora composition of these communities in the north-eastern direction were noted. In the direction of north-eastern border of Carpinus betulus range (ass. Tilio-Carpinetum) and beyond it in oakwoods of Lithuania and Latvia the number of species characteristic of the Querco-Fagetea class reduces as well as species of temperate and temperate-submeridional distribution, while the number of boreotemperate species increases. The species of the last-mentioned group are indifferent from phytocoenological point of view, majority of them are not constant and they do not form the new complex of diagnostic species.