The effects of different seasonal grazing (i.e.summer grazing or winter grazing) on plant composition and diversity in Kobresia humilis alpine meadow were investigated from 2007 to 2011 on the Tibetan plateau using the quadrat method.This study shows that:(1) After long-term different seasonal grazing,the proportion of gramineous functional group reduced and the proportion of the forb functional group increased under both summer grazing and winter grazing;(2)Grazing in different seasons had significant influence on the proportion of different species in the plant community.Compared with no grazing,summer grazing increased the proportion of K.humilis,Taraxacum mongolicum and Potentilla anserina in the community 151.38%,318.87% and 344.15%,respectively,but decreased the proportion of Stipa aliena 41.16%;winter grazing increased the proportion of P.anserina 124.08%,but decreased the proportion of Poa pratensis 45.99%.However,they had no significant influence on the proportion of,Scirpus distigmaticus,Festuca rubra,Elymus nutans,Trigonella ruthenica and Thalictrum alpinum.(3)Generally,the influences of grazing in different seasons on the species richness,diversity index and evenness index of plant communities were not significant but varied with the year.