Abstract: In this paper, with enclosed chamber system, Medicago sativa seedlings were exposed to two CO₂ concentrations levels (ambient and double ambient CO₂ concentration), two temperature levels (ambient and + 2.2℃) and their combination for one growth season in order to research on its biomass accumulation and biomass allocation. The results showed that elevated CO₂ significantly increased the accumulation of root, stem and leaf biomasses and total biomass. And elevated temperature increased the leaf and stem biomasses and total biomass, but had no obvious effect on the root-biomass. Elevated temperature had positive effect on the accumulation of root, stem, leaf and total biomass under elevated CO₂. The combination of elevated CO₂ and temperature increased W_source/W_sink and available biomass, but decreased R/S and root biomass.