AbstractPatterns of biodiversity along environmental gradients is one of the basic issues in biodiversity research. Furthermore, the relationship between species richness and aboveground biomass is one of the most controversial subjects in ecology. In this paper, we studied the patterns of species richness along environmental gradients and the relationship between species richness and aboveground biomass in two major ecosystem types on the Tibetan Plateau, alpine meadow and alpine steppe. The results are summarized as follows: (1) 267 species were recorded in 250 plots of 50 sites, 179 of which occurred in alpine meadows and 135 in alpine steppe. The number of species ranged from 3 to 32 in alpine meadow, while it ranged from 2 to 18 in alpine steppe. (2) Species richness increased with latitude and longitude, and decreased with altitude. A stepwise regression analysis of species richness and environmental factors showed that the species richness was significantly correlated with growing season precipitation and warmth index. (3) Species richness was positively correlated with aboveground biomass.