Rising atmospheric CO2 concentrations stimulate photosynthesis and productivity of C3 plants such as rice (Oryza sativa L.), a phenomenon known as the CO2 fertilization effect (CFE). Previous studies have shown that a greater sink capacity (e.g. grain number, or grain size) enhances the CFE on rice yield and that cytokinin is involved in the sink capacity; however, the direct involvement of this phytohormone in the CFE remains unclear. In this study, we tested whether exogenous application of kinetin improved the CFE of cultivars with different sink sizes (Koshihikari, japonica, with small sink; Takanari, indica with large sink) using a free-air CO2 enrichment experiment on a rice paddy. We hypothesized that kinetin increases the sink size and thereby enhances the CFE, particularly for the cultivar with a smaller sink. Elevated CO2 (200 mu mol/mol above ambient concentration) increased grain yield by 11.4%, averaged across two cultivars. Kinetin significantly improved the CFE in Takanari from 11.2% to 19.2%, but not in Koshihikari. The enhancement of the CFE by kinetin in Takanari appeared through the increased sink size resulting from a greater panicle density. In contrast, kinetin did not increase panicle density or sink size in Koshihikari and failed to enhance the CFE. We therefore conclude that sink size improves CFE on rice yield, and that kinetin could increase sink size, but the effect is cultivar dependent.