We compared the CCSR/NIES/FRCGC coupled general circulation model (GCM) with the atmospheric GCM under control and future warmed conditions to investigate the effects of air-sea coupling on tropical cyclone properties, such as intensity and precipitation. Air-sea coupling suppresses tropical cyclone activity, because water vapor supply is reduced by sea surface temperature (SST) decrease due to the cold wakes under the tropical cyclones. Air-sea coupling tends to suppress tropical cyclone activity to a greater extent in a warmer world, because of the higher base SST and a larger decrease in SST due to the enhanced thermal stratification of the upper ocean. The overestimation of tropical cyclone activity in atmospheric models is more significant for extreme variables such as maximum precipitation than for averaged variables. Therefore, changes in tropical cyclone activity due to global warming based on atmosphere-only models may be overestimated, especially for extreme events.