A series of Ni-Mo-MgO/SBA-15 catalysts (MGS-X, Ni = 13 wt%, Mo = 1 wt%, MgO content = x, x = 0, 1, 3, 5 wt%) were prepared using an impregnation method. The catalysts were characterized by X-ray diffraction, hydrogen temperature-programmed reduction, nitrogen adsorption-desorption, and transmission electron microscopy. It was found that, running counter to expectation, the addition of MgO did not improve stability of the Ni-Mo/SBA-15, while enhancing the oxidation of Mo and decreasing specific surface area, and increasing the rate and amount of shell-like carbon, which has high toxicity. It is concluded that the oxidation of Mo is the main reason for dramatically improved catalyst deactivation.