Abstract: The relations of species diversity of different types of plant communities with elevation, slope orientation and other geographic factors on Sejila Mountain which is situated in southeastern part of Tibet were studied based on plot investigation. The results showed that the layering structure of plant communities in this specific zone was influenced by elevation and slope orientation. The species diversity index of the plant communities on the shady slopes was generally lower than that of on the sunny slopes. The species diversity index of the different growth types of plants inside the forest communities on the shady slopes was in the order: shrub layer > herbaceous layer > arbor tree layer. While the species diversity index of the different growth types of plants inside the forest communities on the sunny slopes was in the order: herbaceous layer > shrub layer > arbor tree layer. The differences in plant species composition and types of plant communities between the shady and sunny slopes decreased along with the increment of altitude, The similar change tendency took place with species richness index, diversity index and evenness index of the plant communities.