The genus Trigonobalanus belongs to a primitive branch of Fagaceae, the trigonobalanoids. Fossil evidence suggests that the trigonobalanoids originated as early as the Paleocene to Eocene, and spread across the Eastern and Western hemispheres, which makes them attractive for biogeographical and phylogenetic studies. Trigonobalanus verticillata was previously only known from tropical Southeast Asia (Sulawesi, Borneo, and Peninsular Malaysia) until we found T. verticillata on Mt. Yinggeling, Hainan Province, a large tropical island of China, during a floral survey in December 2005. We studied the T. verticillata population by plot survey and analyzed age structure, biodiversity of the forest community with T. verticillata, importance rank of tree species, niche breadth, and niche overlap with T. verticillata. The population of T. verticillata was found in the upland areas of Yinggeling between 1100 m and 1400 m, where the communities were dominated by Dacrydium pectinatum，Altingia obovata, Castanopsis carlesii, etc. In two plots of 1500 m², a total of 90 tree and shrub species with diameter at the breast height (D_dbh) greater than 2 cm were recorded, belonging to 47 genera in 29 families. The evenness and Shannon-Weaver diversity indices of the forest community were 0.81-0.85 and 3.20-3.37, respectively. T. verticillata was the 9^th or 10^th most important canopy species in the community. The population of T. verticillata was low in density and unstable with only a few individuals of young seedlings and saplings with Ddbh < 2.5 cm. Its niche breath was 1.65, ranked 19^th in the community. Its niche overlap with other dominant species was usually lower than 0.3. Large areas of upland and relatively undisturbed forest habitats probably account for the existence of T. verticillata population. We suggest preserving the existing community by establishing the Yinggeling Nature Reserve as soon as possible because of low population density, unstable age structure, low niche breath, low niche overlap, and importance as a canopy species.