Abstract: A field experiment using OTC(open-top chamber)was carried out with treatments different in nitrogen supply(0,5,15 g·m^-2) and CO₂ level(350 vs 700 μmol·mol^-1) to investigate effects of the treatments on biomass and root/shoot ratio(R/S) of Calamagrostis angustifolia.Results show that effect of elevated CO₂ on biomass differed in the growth stage.Elevation of CO₂ level stimulated growth of the biomass of the above ground parts of the plant mainly at its early growth stage.It increased the biomass by 12.42%-22.60% at the jointing and heading stages in all the nitrogen treatments,and by 3.11%-12.97% at the dough and maturing stages.The response of the underground parts,however,was obvious at the late growth stages,varying in the range of 17.63%-42.20% except at the jointing stage.The response of biomass and R/S to elevated CO₂ was related to nitrogen level.The promoting effect of elevated CO₂ on biomass and R/S of Calamagrostis angustifolia was only significant in treatments higher in nitrogen supply,while it was not in CK.The stimulating effect of CO₂ on underground biomass is the main cause to increased R/S.