Water absorption behavior and flexural strength properties of carbon fiber reinforced plastics (CFRP) under hot-wet environment were examined. Those of epoxy resin were also examined for reference. Weight gains of CFRP and epoxy resin were measured after immersion in distilled water at temperatures under 90°C. Quasi-static flexural tests of CFRP and epoxy resin were conducted after immersion for 180 days. Weight gains of CFRP and epoxy resin increased with increasing water temperature. After immersion for 180 days at 90°C, weight gain of CFRP became 3.3times higher and that of epoxy resin was 2.3 times higher than that at RT, respectively. When CFRP and epoxy resin were immersed in distilled water at 90°C, weight gains of CFRP and epoxy resin increased and then decreased. Flexural strengths of CFRP and epoxy resin decreased in distilled water at temperatures less than 90°C. Flexural strengths of dried CFRP and epoxy resin after immersion recovered but were lower than that of virgin CFRP and epoxy resin. Debonding of fiber/resin interface and crack initiation in epoxy resin in distilled water resulted in the strength reduction.