Soil microbial biomass and enzyme activities have an important influence on nutrient cycling. The temporal variation in soil microbial biomass C, N, and enzyme activities during the growing season were determined under three different alpine grasslands in Northern Tibet. The results showed that soil microbial biomass C, and N contents and enzyme activities of the alpine meadow (AM) and the alpine meadow steppe (AMS) sites were much higher than those of the alpine steppe (AS) site. Soil microbial biomass C, N variations were not significantly correlated with the soil temperature and moisture, except that microbial biomass N seemed associated with the microsite where soil temperature was higher. Our results demonstrate that soil temperature was one of most important factors explaining the seasonal variation of microbial biomass N, but how the alpine grassland ecosystem's type affects microbial biomass C, N and enzyme activity are still needed to be clarified by determining other correlative ecological factors and covering prolonged observation periods.