Rising atmospheric CO2 concentration stimulates C3 plant photosynthesis and productivity, known as the CO2 fertilization effect (CFE). Improving the CFE on C3 crops may enhance grain yield. Through a free-air CO2 enrichment (FACE) experiment, the response of rice yield to elevated CO2 concentration (EC) in combine with exogenous application of a plant regulator: kinetin (KIN) was checked. During early tillering stage, KIN was sprayed to Koshihikari (Japonica) and Takanari (Indica) grown at both of ambient CO2 concentrations (AC) and EC. Rough rice yield was stimulated by 11.4% by EC averaged across two cultivars, moreover, KIN significantly improved the yield enhancement by EC in Takanari. However, KIN negatively affected the EC-enhanced yield in Koshihikari. Spikelet and panicle density responded to EC+KIN similarly with yield. KIN marginally stimulated the grain filling percentage and the CFE on percentage of ripened spikelet. These results suggest that exogenous application of KIN could increase the sink capacity, stimulate the biomass allocation, thus improve the CFE on rice yield.