Floristic composition of the Fagus japonica communities and their habitats were investigated in summergreen broad-leaved forests in the Tanzawa Mountains. The communities were all observed on upper sideslopes above erosion fronts and were classified into four associations based on phytosociological method. Three of the associations were mainly distributed in the West Tanzawa Mountains, each with different vertical distributions, as follows : Styraco shiraianae-Fagetum japonicae (400 〜 900 m) ; Fagetum crenato-japonicae (800 〜 1200 m) ; Corno-Fagetum crenatae (subassociation of Fagus japonica) (1050 〜 1300m). The fourth association was Veronico-Fagetum crenatae (subassociation of Fagus japonica) (1050 〜 1300m) and was distributed in the East Tanzawa Mountains. In this area, since the understory has been changed through the impact of Sika deer (Cervus nippon), the association was characterized by plants disliked by Sika deer. Fagetum crenato-japonicae was classified into two subassociations : typical subassociation and subassociation of Hydrangea hirta. Veronico-Fagetum crenatae (subassociation of Fagus japonica) was classified into two variants : typical variant and variant of Pterostyrax hispida. All four under units varied in their species composition, paralleling changes in their microtopography on the upper sideslope.