Abstract: The species composition, height, diameter class structure and tree species diversity were examined in a 1 hm2 plot in old-growth tropical montane rain forest in Bawangling Nature Reserve of Hainan Island. The tree species richness and diversity indexes were high relative to forests in mainland China. Both the numbers of tree species and tree density decreased as height class or diameter class increased, fitting both negative exponential and negative power equations. There was a significant positive correlation between tree species abundance and tree density for trees of different height classes, different diameter classes and for trees in different quadrats in the tropical montane rain forest. At successional maturity, only a small number of species with a small proportion of individuals could reach the canopy layer in the old-growth community.