Abstract Flame front structure characteristics of turbulent premixed flames including the local radius of curvature, fractal inner cutoff scale and local flame angle were calculated from the experimental OH-PLIF images of CH4/air flames and those diluted with CO2 and superheated H2O at 0.5 MPa and 573 K. The convex and concave structures of the flame front were detected and statistical analysis including PDF and ADF of the local radius of curvature and the local flame angle was conducted. Results showed that the flame front of turbulent premixed flames at high pressure and high temperature is a wrinkled flame front with small scale convex and concave cusps superimposed on large scale branches, while the whole flame front tends to be convex to the unburned mixture. The effect of EGR gas dilution on the flame front structure of turbulent premixed flames is dominated by CO2 rather than H2O dilution. The quantitative flame front characteristics reveal the complicated effect of CO2 dilution on turbulent premixed flames characteristics observed in our previous research. In the case of CO2 dilution, some local wrinkled structures become sharp and propagate deep into the burned mixture, resulting in a decrease of the fractal inner cutoff scale, the formation of a thick flame brush and the enlargement of the mean flame volume, while the overall wrinkled scale increases significantly due to the suppression of the concave structure with CO2 dilution, leading to a decrease of the turbulent flame front area, and subsequently to a decrease of ST/SL.