It is urgent to develop new efficient ammonia synthesis catalysts using non-precious metals. Herein, the Mo2C species is introduced into a carbon-supported Mo catalyst by in situ carburization of a carbon-supported Mo catalyst in H2. In combination with the presence of the Mo2C phase as well as the enhancement of the graphitization degree of carbon and the amount of the low-valent Mo species, the migration and the exchange of the adsorbed species with the gaseous species are accelerated. As a result, the catalyst with carbonization treatment shows higher ammonia synthesis activity than the sample without carbonization, and the ill effect of the poisoning of reagent gases also is alleviated.