Abstract: The research was made on the structures of the size class and height class of the tree layer species of the subtropical mixed evergreen and deciduous forest in a fixed quadrat of one ha.and working out the tree species diversity and evenness of the arbors according to the size class.The results showed that the tree species numbers and individual numbers of the arbors in the community declined as the growing of the size class and the height class,and there was a remarkable positive correlation between the tree species number and individual number.The sum of basal area at DBH (Diameter at breast height) fluctuated as the increase of size class,though it produced a curve which is composed of a former ascending part and a latter descending part as the increase of the height.The diversity index and evenness of the community were relatively high,so there were no distinct dominant populations in the community.The abundances of each species varied slightly,therefore the community has a steady structure.The rare plants of the community had both a relatively high tree species number and individual number.The seedlings of the arbors abound in the community especially the seedlings of the rare and endangered plant species,the community is in a good state of regeneration.Finally,the tree layer can be divided into three parts.