Abstract: Aboveground biomass and community stricture at 40 sites with grazing exclusion were surveyed on a west-east alpine grassland transect along precipitation gradient in Northern Tibetan Plateau to evaluate effect of growing season rainfall on both aboveground biomass and species diversity and to discuss the relationship between aboveground biomass and species diversity indexes. The precipitation pattern significantly affected species richness, Shannon-Wiener diversity and Pielou evenness indexes of the alpine communities in inner Northern Tibetan Plateau and that community structure related closely with primary productivity. All of aboveground biomass, species richness, Shannon-Wiener diversity index and Pielou evenness index exponentially increased with growing season precipitation addition. In the relationship between species richness and aboveground biomass of alpine grasslands, the discriminant coefficient of the hump-shaped model (R²=0.754) was slightly higher than linear regression (R²=0.743), however, species richness monotonically increased with aboveground biomass of alpine grasslands along Northern Tibetan Plateau Transect without any monotonic decreasing interval in the hump-shaped model. Moreover, hump-shaped model in our results indicated that species richness more likely showed a monotonic decreasing with aboveground biomass when it was more than 121.17 g/m² and to make this humped relationship to be standard. Hump-shaped models described both Shannon-Wiener biodiversity and Pielou evenness index with aboveground biomass that decreasing intervals were relatively narrower than increasing ones and peak values took place at aboveground biomass of 71.90 and 60.90 g/m² respectively.