With Leymus chinensis-herbage and Stipa baicalensis-Filifolium sibiricum communities as deputies, the sampling intensity of species diversity of grassland communities was studied in the Songnen Plain of northeast China. The results of nested quadrat showed: the minimal area of these two communities was 1/4～1/2 m². The results of Pielou's pooled quadrats showed: the minimal number of 1, 1/4 and 1/6 m² of square quadrats was or so 12, 22 and 28 respectively in L. chinensis-herbage community, and 10, 18 and 25 in S. baicalensis-F. sibiricum community. The above three types of area got unanimus results in the indices of diversity at 95% of probability level. A large number of small quadrats was preferable to a small number of large quadrats.