Effect of rotational grazing of yak in two (warm and cold) seasons for over 20 years on species diversity and biomass of plants was investigated in Potentilla fruticosa rangeland in northern Qinghai-Tibetan Plateau. In the paddock grazed in warm seasons (WSP), 37 species (Gramineae : Cyperaceae : forbs : shrubs = 6 : 5 : 23 : 3) appeared and 44 (Gramineae : Cyperaceae : forbs : shrubs=7 : 6 : 27 : 4) did in that grazed in cold seasons (CSP). WSP was smaller in values of Richness, Shannon-Wiener, Simpson and Pielou evenness indices than CSP. Vegetation coverage and community height were significantly lower in WSP than in CSP (p<0.01). Number of plant species was significantly smaller in WSP (7-16 species/m^2) than in CSP (17-22 species/m^2) (p0.001). Aboveground biomass of Gramineae was significantly lower in WSP than in CSP (p<0.01). Subdominant species in WSP were Kobresia humilis, Leontopodium nanwn and Potentilla anserina which were short in height and grazing-resistant, while those in CSP were Poa pratensis, Elymus nutans and Kobresia capillifolia which were tall in height. It was considered that long-term rotational grazing of yak in warm and cold seasons decreased floral diversity and biomass of plants, especially in Gramineae in WSP as compared with those in CSP, and the deterioration was going on in WSP.

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