Abstract: Measurements of species diversity of plant communities along both DCCA axis 1 and an altitudinal gradient in the northern slopes of Qilian mountains was carried out. The results showed that: 1) Both species richness and diversity in the herb layer significantly peaked at the intermediate elevations.The same trend was also suggested in the shrub layer, but the curves of both richness and diversity were not significant. Both richness and diversity in the tree layer showed no changes with increased altitude, suggesting that the order of sensitivity in terms of the responses of richness and diversity to altitudinal gradients was herb layer, then shrub layer and tree layer. 2) Evenness of the three layers showed no noticeable trend with altitude, suggesting that the dynamics of evenness of a given plant community may be strongly influenced by some fundamental characteristics of plant community rather than resource availability. 3) The fitted curve of both diversity and richness of herb layer with DCCA axis 1 was more significant than that fitted with altitudinal gradient, while that of shrub layer showed the opposite trend. 4) Of all the comparisons between shrub and herb layer in terms of richness, diversity and evenness, the differentiation was significant for both richness and diversity in the lower altitudinal zone ( p <0.01), the mid lower to mid-altitudinal zone ( p <0.01)(excluding Simpson diversity index ( D ) and in the high altitudinal zone ( p <0.05)(excluding both Shannon Wiener diversity index ( H ′) and Simpson diversity index). The differentiation of evenness between shrub layer and herb layer was not significant in any of the comparisons. The pattern of species diversity altitudinal gradient reported here demonstrates a monotonic relationship between productivity and species diversity related to resource availability.