The reduction of CO2 emission by the transport sector is necessary to be realized the low carbon society. In the near future, further CO2 emission reduction is expected by the diffusion of PHEV. The aim of this study was to evaluate the potential of PHEV to reduce CO2 emission based on real-world driving data (probe car data) and simulation. The probe car data of 35 conventional HEVs from April to August in 2011 were analyzed. The type of simulated PHEV system was all electric range, which operated only by battery power as long as available battery capacity was remaining (EV mode), and then operated like conventional HEV after battery was depleted (HEV mode). Charging frequency was once a day at home after midnight as a realistic scenario. The results showed that the travel distance of 43% was converted to EV mode, and the gasoline consumption was reduced by 44%. The CO2 emission was totally reduced by 17% considering electric power consumption. CO2 emissions of each vehicle were reduced by 1-44%. CO2 reduction amount of each vehicle varied widely reflecting their each own ways of car use and operating conditions. It is indicated that the diffusion of PHEV is a realistic and efficient measure to reduce CO2 emissions in consideration of actual car use and operating conditions. Furthermore, low carbon power supply as well as diffusion of PHEV is more effective to CO2 reduction.