beta-Mo2N0.78 has been prepared via the temperature-programmed reaction of MoO3 under N-2-H-2 mixture gases with lower purity. As a hydrodesulfurization (HDS) catalyst,beta-Mo2N0.78 was studied and its characteristics before and after HDS reaction were analyzed by X-ray diffraction analysis, scanning electron microscopy, elemental analysis, and Mo K-edge extended X-ray absorption fine structure analysis. These results supported the following conclusions: (1) both prereduction and pre-sulfiding did not improve the activity of the catalyst; (2) the consumption of surface oxygen species resulted in an activity decrease at the initial stage of reaction, and the following sulfidation of the catalyst made the activity constant; (3) the second nitriding treatment could renew the initial activity of the catalyst after HDS reaction.