Many combustion systems use the technique called exhaust gas recirculation. Thus, the effects of N-2/CO2 dilution on the combustion characteristics have been of interest. In this study, the dilution effects on the flame propagation velocity (FPV) of an oxy-methane premixed mixture were investigated using a state-of-the-art annular stepwise diverging tube. The relationship between FPV and the length scale was measured for various equivalence ratios in the dilution ratio range 65-75% N-2 and 50-65% CO2. The characteristic variations of FPV in each dilution case could be described as a surface in a concentration-length-velocity diagram, and this provides a bird's eye view of the dilution effects on flame propagation. Two distinctive scalar values of the quenching distance and the critical FPV were investigated. At the same dilution ratio, CO2 dilution caused more significant variation than N-2 dilution did regarding FPV, quenching distance, and flammable limits. In particular, the critical FPV was compared with the laminar burning velocity (LBV) calculated using a PREMIX code employing GRI-3.0. In addition, fictitious species (F-N-2/F-CO2) were used in the reaction mechanism to distinguish chemical effects. Conclusively, results showed that CO2 dilution reduced LBV significantly not only by its larger thermal capacity but also by its active involvement in the chemical reaction.