Plant species and composition characteristics were investigated in Phragmites communis community and Kobresia tibetica community． The results show that Phragmites communis and Kobresia tibetica were the dominant and the subdominant species in Phragmites communis community，with important values of 0． 434 2 as in sum． Kobresia tibetica was the dominant species in Kobresia tibetica community，with an important value of 0． 187 6． Species diversity analysis showed that in the two communities，the Shannon-Wiener index(H) and the Pielou(evenness) index(J)varied with large amplitudes of fluctuation，while the Simpson(dominance) index(D) showed much less variations．The diversity index was significantly correlated positively with the evenness index and negatively with the ecological dominance index，while not with the species richness index(S)． The H，D，J and S values were not significant different in the two communities，each consisted of 14 species． The similarity coefficient between the two communities was 51． 85%． Thus，the community composition of Phragmites communis community was closely related with that of Kobresia tibetica community．