Abstract: The surface structure and the valence change on sulfided MoO_3/γ-Al_2O_3 catalyst samples were investigated at different sulfidation conditions and before or after methanation reaction using XPS, ESR and TEM methods. The main results were obtained as following. The most of Mo~(+6) were easily reduced to Mo~(+4) during the samples were sulfided. Very small amount of Mo~(+5) existed and decreased when the sulfidation time was increased. More amount of Mo~(+5) was only detected by XPS on the samples of lower Mo content at lower reduction temperature and at shorter sulfidation time. The major valency of sulfur was -2 during the sulfidation proceeded. A small amount of high valence sulfur was detected and vanished during prolongation of sulfidation time. No change of the valence of Mo or S was observed after reaction for 8 hrs. It was demonstrated by TEM photogragh and diffraction pattern that MoS_2 microcrystal grew up remarkably on the sample surface after methanation reaction.