Mung bean (Vigna radiata L.) has the potential to establish symbiosis with rhizobia, and symbiotic association of soil micro flora may facilitate the photosynthesis and plant growth response to elevated [CO2]. Mung bean was grown at either ambient CO2 400 μmol mol–1 or [CO2] ((550±17) μmol mol–1) under free air carbon dioxide enrichment (FACE) experimental facility in North China. Elevated [CO2] increased net photosynthetic rate (Pn), water use efficiency (WUE) and the non-photochemical quenching (NPQ) of upper most fully-expanded leaves, but decreased stomatal conductance (Gs), intrinsic efficiency of PSII (Fv´/Fm´), quantum yield of PSII (ΦPSII) and proportion of open PSII reaction centers (qP). At elevated [CO2], the decrease of Fv´/Fm´, ΦPSII, qP at the bloom stage were smaller than that at the pod stage. On the other hand, Pn was increased at elevated [CO2] by 18.7 and 7.4% at full bloom (R2) and pod maturity stages (R4), respectively. From these findings, we concluded that as a legume despite greater nutrient supply to the carbon assimilation at elevated [CO2], photosynthetic capacity of mung bean was still suppressed under elevated [CO2] particularly at pod maturity stage but plant biomass and yield was increased by 11.6 and 14.2%, respectively. Further, these findings suggest that even under higher nutrient acquisition systems such as legumes, nutrient assimilation does not match carbon assimilation under elevated [CO2] and leads photosynthesis down-regulation to elevated [CO2].