Reaction mechanism between MoO2 powder and NH3 in the temperature range of 1028-1273 K has been investigated. The results show that reduction products are strongly dependent on the reaction temperature employed. It is found that molybdenum nitride (Mo2N) can be successfully synthesized when the temperature is below 1078 K, with the morphologies keep the same as that of the raw material MoO2. Although the temperature is above 1078 K, metal Mo powder will be turned up; at the even higher temperatures, the thermodynamically stable phase will be metal Mo. In addition, MoOxN1-x as an intermediate product is formed during the reduction processes.