Abstract: On the basis of investigation on gaps in the tropical montane rain forest of Bawangling Nature Reserve, Hainan Island, regneration niche and related characteristics were analyzed. The results showed that dominances of the same species in gaps of different sizes and developmental stages were different. And dominances of different species in gaps of the same sizes and same developmental stages were also different. According to values of niche breadth based on gap sizes, the species were classified into 4 types, i. e. the wide, middle, comparatively narrow and narrow amplitute of niche. According to values of niche breadth based on gap age classes, the species in the tropical montane rain forest were classified into 4 types, i. e. efficient utilization on gap resources comparatively efficient utilization on gap resources not efficient utilization on gap resources, and very few utilization on gap resources. The non-uniformity of species utilization on resources in gaps of different sizes and age classes indicate that niches based on gap sizes and gap age classes for different species were comparatively separated. Through the formation and development of different sized gaps, tree species satisfied their life history process requirements, so that many species can coexist consistantly in the tropical montane rain forest. Calculations on the dominances and niche breadth of tree species in gaps of different sizes and age classes showed that there are a great proportion of low density, narrow niche species in the tropical montane rain forest. These species have a high dependence on whole ecological environment in the tropical forest ecosystem. The probabilities of local extinction are high once the environment changed. The existence of vast quantity of low density, narrow niche species in the forest indicates that the tropical montane rain forest ecosystem is a vulneralbe ecosystem type in terms of species diversity. Great attention should be paid to the diversity conservation of the low density, narrow niche species when any management measures are taken to the tropical montane rain forest.