AbstractMt. Jianfengling, located in southwestern Hainan Island, is one of few areas in China where there exist wellprotected primary forest types resembling tropical rainforest. An 1 hm2 of permanent plot was established in a mountain rainforest and all trees with DBH≥5.0 cm were measured to study community structures and species richness of the forest type. A total of 171 species belonging to 93 genera and 52 families were identified, and a total of 1099 individuals were recorded, of which 1024 were trees. Diversity indices measured as ShannonWiener index (H′), evenness index (E) and Simpson index (D) were 4.11, 0.80 and 0.97 respectively. These indicies varied with sample size and initial DBH of measured individuals. H′, E and D clearly increased with increasing sample size, but exhibited no obvious pattern above 4000 m2 for H′ or 2000 m2 for E and D. As the initial DBH increased, numbers of species and families, as well as H′ and D decreased sharply, while E increased. DBH class frequency distribution revealed that number of individuals decreased with increasing DBH classes following an exponential function, and that community structure could be separated into four strata along an elevation gradient, with individuals distributed uniformly or randomly in each layer. The orientation of mountain rainforest of Jianfengling in the world rainforests is discussed from three aspects of climatology, family and genus composition, and comparison of species richness between rainforests of Jianfengling and other tropical regions. Climatological analysis indicated that the mountain rainforest of Jianfengling thermally corresponds to subtropical/warmtemperate zones and to arid/humid ranges of perhumid climate (rainforest climate). Floristically, subtropical/warm-temperate elements dominated, especially Fagaceae and Lauraceae (together accounting for more than 34% of the total importance value), while tropical elements were subordinate. Compared with the world tropical forests, species richness in the mountain rainforest in Jianfengling was lower than typical tropical rainforests but somewhat higher than tropical forests with low rainfall. Thus, the mountain rainforest of Jianfengling is much different with typical tropical rainforest and is a transitional forest type from tropical to subtropical/warmtemperate rainforests. In addition, two measures of species richness, mean population density (MPD) and number of speciesnumber of individuals relationship, are proposed and mathematically defined.