This paper conducted experimental and numerical studies to evaluate the performance of dimethyl methylphosphonate (DMMP) in suppressing methane/air premixed flames. Laminar burning velocities were measured using the Bunsen flame method at an elevated temperature (373 K). As the concentration of DMMP increased, the laminar burning velocity initially decreased quickly and then gradually, which indicated the appearance of the saturation effect of the inhibition efficiency. The saturation effect was especially obvious for flames at an equivalence ratio of 1.2. Numerical calculations coupling with detailed chemical reaction mechanism were conducted to interpret the measurements. The newly constructed mechanism predicted well the laminar burning velocities. Sensitivity analyses revealed that as the equivalence ratio increased, reactions involving high-valence phosphorus had diminishing influence on the laminar burning velocity but the reactions involving low-valence phosphorus became more influential. By comparing the thermal and chemical effects of the main phosphorus-containing reactions in the rich flames (phi=1.2), it was found that the thermal effect increased while the chemical effect decreased with the increase of DMMP addition. Therefore, the saturation effect of the inhibition efficiency of DMMP at high loadings was partially attributed to the heat release of the recombination reactions involving small P-bearing species.