Abstract: Hydroponic experiments were carried out to study the mechanisms of CO2 enrichment-induced decrease of nitrate accumulation in two genotypes of Chinese cabbage (low nitrate accumulation genotype Dongfei and high nitrate accumulation genotype Gaoxiongtiancui). Results showed that elevated CO2 not only significantly increased the biomass and nitrate reductase (NR) activity, but also significantly decreased the nitrate concentration in leaves, stems and roots of both genotypes. In addition, nitrate uptake by plants was also enhanced by elevated CO2, and the elevated CO2-increased percentage of nitrate uptake was higher than that of biomass, indicating that not only the dilution effect resulted by biomass increase, but also the change of physiological processes are the reasons leading to the decrease of nitrate accumulation in plants under elevated CO2. Moreover, the decreased percentage of nitrate concentration was correlated well with the increased percentage of NR activities in Dongfei genotype, while in Gaoxiongtiancui genotype the decreased percentage of nitrate concentration was greatly correlative to the increased percentage of biomass. These results demonstrate that the increases of both NR activity and biomass are the pivotally mechanisms reducing nitrate accumulation in vegetable under CO2 enrichment, whereas distinct genotypic differences exist in the contribution rates of these two mechanisms.