We described the tree species diversity and floristic composition of a tropical seasonal rainforest located in Xishuangbanna, south-west China, based on a census of all trees with diameter at breast height (dbh) ≥ 10 cm in three 1-ha plots. A total of 1283 stems of 53 families, 125 genera and 207 species were recorded in these plots. Families with greatest importance value were Euphorbiaceae, Meliaceae, Sapindaceae, Lauraceae and Lecythidaceae. Lauraceae was the most species-rich family. Forest structure was marked by few tall emergent trees, such as Pometia tomentosa (Sapindaceae). The average number of large trees (dbh ≥ 70 cm) was 15 stems ha-1. Beta diversity indices indicated that tree species composition differed greatly among the three plots. Our results revealed that species diversity of tropical seasonal rainforests in Xishuangbanna was lower than that of tropical lowland rainforest in South-East Asia. This may be due to lower annual precipitation, higher elevation and more distinct seasonality in Xishuangbanna. Additionally, tree species composition in our study area differs from that of the dipterocarp forests in South-East Asia. This study will help us to understand the patterns of tree species composition and diversity in the northern edge of tropical Asia.