The alpine grasslands on the Tibetan Plateau are sensitive and vulnerable to climate change. However, it is still unknown how precipitation use efficiency (PUE), the ratio of ANPP to precipitation, is related to community assembly of plant species, functional groups or traits for the Tibetan alpine grasslands along actual environmental gradients. We conducted a multi-site field survey at grazing-excluded pastures across meadow, steppe and desert-steppe to measure aboveground biomass in August, 2010. We used species richness, the Shannon diversity index, and cover-weighted functional group composition (FGC) of 1-xerophytes, 2-mesophytes, and 3-hygrophytes to describe community assembly at the species level; and chose community-level leaf area index (LAI c ), specific leaf area (SLA c ), and species-mixed foliar δ 13 C to quantify community assembly at the functional trait level. Our results showed that PUE decreased with increasing accumulated active temperatures (AccT) when daily temperature average is higher than 5°C, but increased with increasing climatic moisture index, which was demined as the ratio of growing season precipitation (GSP) to AccT. We also found that PUE increased with increasing species richness, the Shannon diversity index, FGC and LAI c ,decreased with increasing foliar δ 13 C, and had no relation with SLA c at the regional scale. Neither soil total nitrogen nor organic carbon has no influence on PUE at the regional scale. The community assembly of the Shannon index, LAI c and SLA c together accounted for 46.3 % of variance in PUE, whilst climatic moisture index accounted for 47.9 % of variance in PUE at the regional scale. This implies that community structural properties and plant functional traits can mediate the sensitivity of alpine grassland productivity in response to climate change. Thus, a longterm observation on community structural and functional changes is recommended for better understanding the response of alpine ecosystems to regional climate change on the Tibetan Plateau. 16 Abstract The alpine grasslands on the Tibetan Plateau are sensitive and vulnerable to climate change. However, it is 17 still unknown how precipitation use efficiency (PUE), the ratio of ANPP to precipitation, is related to community 18 assembly of plant species, functional groups or traits for the Tibetan alpine grasslands along actual environmental 19 gradients. We conducted a multi-site field survey at grazing-excluded pastures across meadow, steppe and desert-20 steppe to measure aboveground biomass in August, 2010. We used species richness, the Shannon diversity index, 21 and cover-weighted functional group composition (FGC) of 1-xerophytes, 2-mesophytes, and 3-hygrophytes to 22 describe community assembly at the species level; and chose community-level leaf area index (LAI c ), specific leaf 23 area (SLA c ), and species-mixed foliar δ 13 C to quantify community assembly at the functional trait level. Our results 24 showed that PUE decreased with increasing accumulated active temperatures (AccT) whe...