Abstract: Based on a comparison of fragmented and primary tropical rain forests in Xishuangbanna, the effects of fragmentation on structure, species composition, life form, tree populations, plant abundance and species diversity of the tropical rain forest are analyzed. With increasing fragmentation, edge effects and disturbances, fragments become different from the primary rain forest in various ways. Compared with the primary forest, the fragments were incomplete in community structure, lower in plant abundance and species diversity, and showed a relatively lower percentage of epiphytes and chamaephytes but a conspicuously higher percentage of lianas. In the fragments with slight to intermediate disturbances, tree populations of only 1 or 2 individuals make an increasing percentage of the total, indicating that the populations of many tree species have become more unstable. In the fragments with serious disturbances, tree species with only 1 or 2 individauls present disappeared first. As the fragments in Xishuangbanna formed and developed under strong human impact, their species diversity shows complicated changes. Liana species diversity increased with a certain extent of disturbances, but decreased when over disturbed. Species diversity decreased in general with increasing disturbance.