The reaction of ultrafine spherical β-MoO3 powders with CO or the mixed CO-CO2 gases by the temperature-programmed reaction (TPR) method is investigated in order to produce Mo2C. From the experimental results, it is concluded that the reaction process between MoO3 and CO is composed of two stages: the reduction of MoO3 to MoO2 and the carburization from MoO2 to Mo2C. Mo4O11 as an intermediate product is formed during the first reduction stage (from MoO3 to MoO2), which is similar to the reduction process of MoO3 to MoO2 by H2. The product Mo2C always keeps the same platelet-shaped morphology as that of MoO2. In addition, it is found that adding a certain proportion of CO2 into the reducing gases has a large inhibiting effect on the carbon deposition reaction of CO, which is beneficial for the preparation of pure Mo2C. However, if the proportion of CO2 is too high, Mo2C cannot be obtained and the product will turn to be MoO2 instead.