Net photosynthetic rate (P N ) of leaves grown under free-air CO 2 enriched condition (FACE, about 200 μmol mol -1 above ambient air) was significantly lower than P N of leaves grown at ambient CO 2 concentration (AC) when measured at CO 2 concentration of 580 μmol mol -1 . This difference was found in rice plants grown at normal nitrogen supply (25 g m -2 ; NN-plants) but not in plants grown at low nitrogen supply (15 g m -2 ; LN-plants). Namely, photosynthetic acclimation to FACE was observed in NN-plants but not in LN-plants. Different from the above results measured in a period of continuous sunny days, such photosynthetic acclimation occurred in NN-plants, however, it was also observed in LN-plants when P N was measured before noon of the first sunny day after rain. Hence strong competition for the assimilatory power between nitrogen (N) and carbon (C) assimilations induced by an excessive N supply may lead to the photosynthetic acclimation to FACE in NN-plants. The hypothesis is supported by the following facts: FACE induced significant decrease in both apparent photosynthetic quantum yield (Φ c ) and ribulose-1,5-bisphosphate (RuBP) content in NNplants but not in LN-plants.Additional key words: apparent quantum yield; carboxylation efficiency; net photosynthetic rate; nitrogen supply; Oryza; ribulose-1,5-bisphosphate regeneration.