Abstract: Leymus chinensis–one of the most wide spread dominant species in the grassland of northeastern China. In this paper, the regularities of formation of its aboveground biomass was studied. Every year, in early April L. chinensis’ population turned green. Its highest relative growth rate (R), 0.l142g·g-1· day-1, occurred before earing (the second ten days--the last ten days of May)) but its highest absolute growth rate (G), 7.46g· m-2·day-1, appeared during florescence (the second ten days--the last ten days of June). It was found that the R was mainly controlled by the growth regularity of L. chinensis itself, while the G was influenced by the rainfall and temperature of growing season. The aboveground biomass of L. chinensis’ population changes with time and it reaches its highest peak at the growing season. The mean highest value of biomass was 305g·m-2. It occurred during the second ten days of August and then slowly declinded. The aerial parts of L. chinensis’ population all died during the second ten days of October. From our observation, the rainfall in earlier stage (April--July) of growing season was the main limiting factor to the aboveground gross biomass. The height of the peak of its aboveground gross biomass depends on the growth of the biomass of assimilating tissue of L. chinensis’ population. The increase of the aboveground gross biomass positively correlats with the growth of the assimilating tissue (r=0.9472).