Abstract: The belowground biomass and its relationships with environmental factors of natural grassland: alpine steppe, mountain meadow, mountain meadow steppe, mountain steppe and mountain-desert grassland of the northern slopes of the Qilian mountains were investigated. 1) The belowground biomasses of five kinds of grassland differed significantly (P＜0.05) except for mountain steppe, mountain meadow steppe, and alpine steppe. The belowground biomass reduced with soil depth in an exponential manner and with a “T” shape distribution. 2) Seasonal change of the belowground biomass was of a “W” type in alpine steppe and of a “N” type in other grasslands, and decreased with soil depth. 3) Below ground net productivity of mountain meadow was the highest (546.84 g/m²), and was followed by mountain meadow steppe (410.76 g/m²), mountain steppe (358.12 g/m²), alpine steppe (301.33 g/m²) and mountain-desert grassland (81.68 g/m²). Turnover value of the belowground biomass was beyond 45%. 4) There were negative correlations between the belowground biomasses of five kinds of grassland and the hydrothermal factor, though the correlations were not significant except for mountain-desert grassland. But the correlations (positive or negative) between the belowground biomasses of five kinds of grassland and the hydrothermal factor were highly significant in the 7-8 months with better hydrothermal conditions. In other months, the correlations between the belowground biomasses of different grasslands and environmental factors was very varied.