AbstractTheα diversity of four Stipa communities , i. e. S . baicalensis , S . grandis , S . krylovii , and S . klemenzii in the Inner Mongolia Plateau were studied using comparable plot methods. The results showed that the species number of four Stipa communities were affected by both the size and shape of quadrats. The species-area relationship could be characterized by a power function model , S = aln A + b. The difference between numbers of species obtained by oblong quadrats and by square quadrats with twice the size of the oblong quadrats was not significant . At the same scale the species number decreased along the ecological gradients from the S . baicalensis community to the S . klemenzii community. The richness indices , including species number of the communities and Margalef index , also declined from the S . baicalensis community to the S . klemenzii community. Diversity indices including Shannon-Wiener index and Hill’ s indices N1 and N2 , were also reduced.