Plant communities in the Qilian Mountains were studied to see how species diversity changes with altitude. The herb layer had the highest species richness and diversity at middle elevations, while the tree layer showed no change. Evenness of the layers did not vary with altitude. The relationship between species diversity and altitude was stronger for herbs than for shrubs. Shrub and herb layers differed significantly in richness and diversity at different altitudes. Overall, species diversity is linked to resource availability in these mountain ecosystems.