Molybdenum carbides have been believed to catalyze C?C coupling to generate aromatics from methane conversion. Herein, a systematic study of methane activation on the Mo-terminated Mo2C surfaces have been performed using density functional theory calculations. Our results indicate that the Mo-terminated ?-Mo2C surface exhibits a superior activity toward methane activation. Carburization through insertion of carbon atoms into the subsurface interstitial sites of ?-Mo2C reduces the reactivity towards methane activation; and complete carburization in the subsurface layer of ?-Mo2C makes it behave similarly to ?-Mo2C, which has a completely carburized subsurface layer, and ?-MoC towards methane conversion. Consequently, dimerization of the CH* adspecies through C?C coupling occurs more readily, resulting in C2H2*, a potential precursor to produce long chain hydrocarbons or aromatics. This study also demonstrates a dynamic nature of carburization of molybdenum carbides under the condition of methane activation, which is expected to play an important role in determining the activity and selectivity for, e.g., dehydro-aromatization.