Abstract Methane/air turbulent premixed flames diluted with superheated water vapor at high-pressure and high-temperature were experimentally investigated to explore the effects of recycled water vapor on turbulent flame characteristics from the viewpoint of applying high-temperature air combustion (HiTAC) to high-load combustors as well as elucidating those effects of exhaust gas recirculation (EGR) in IC engine. A newly devised water evaporator was installed in a high-pressure chamber and superheated water vapor was successfully supplied to air up to 1.0 MPa and 573 K. The maximum dilution ratio defined as the ratio of the molar fraction of H 2 O to those of air and H 2 O was 0.1. Turbulent burning velocity, mean volume and the structure of the turbulent flame region were compared with those of flames diluted with CO 2 , reported previously, which is another major species in recycled burnt gas. Results showed that the effects of superheated water vapor dilution on turbulent burning velocity, S T , normalized by laminar burning velocity, S L , was much weaker than that of CO 2 dilution. The mean volume of the turbulent flame region defined as the region between 〈 c 〉 = 0.1 and 〈 c 〉 = 0.9, was scarcely changed either. This means that the effects of recycled burnt gas on the structure of turbulent premixed flames at high pressure and high-temperature is predominated by CO 2 but not by superheated water vapor, indicating that suppression of combustion oscillation in premixed-type gas-turbine combustors by extension of the volume of heat release region is due to recycled CO 2 . The emission indices of CO and NO x were also measured at high pressure, and it was proved that water vapor dilution is effective to restrain CO emission, which is a possible defect of HiTAC when it is applied to gas-turbine combustors.