The increased supply of photosynthate from maternal tissue is known to promote grain growth in several crop species. However, the effect of increasing photosynthate supply on grain growth receives little attention in rice. This study was aimed at evaluating the effect of increasing photosynthate supply through CO2 enrichment (650 μl I-1) on grain growth in three rice cultivars differing in grain size. CO2 enrichment was applied to the pot-grown plants between anthesis and final harvest. The results indicated that high CO2 treatment enhanced the CO2 exchange rate of leaf tissue, and subsequently increased the sucrose level of peduncle exudate, but it did not promote starch accumulation in the developing grains. This phenomenon was linked to the poor CO2 responses for the grain activities of sucrose synthase, UDP-glucose pyrophosphorylase. ADP-glucose pyrophosphorylase, and starch synthases involved in the conversion of sucrose to starch. Significant cultivar differences also existed for the activities of sucrose to starch conversion enzymes with larger grain size cultivars tending to have higher enzymes activities (expressed on a grain basis), resulting in a greater carbohydrate accumulation.