Abstract: Ni/γ-Al_2O_3, Ni/γ-Al_2O_3-La_2O_3 (La_2O_3 6.7%)and Ni/γ-Al_2O_3-La_2O_3 (La_2O_3 11.7%) catalysts were prepared by incipient wetness technique. The particle size distribution (Fig.9) and the average size of nickel in these catalysts were determined by Transmission Electron Microscopy (TEM). Average particle size could be compared with the data determined by X-Ray Diffraction (XRD)(see Table 3). It has been found that the particle size of nickel in catalysts is decreasing (Table 1) and activity of methanation is increasing (Fig.2) as adding La_2O_3 to Nicatalyst. Results of TEM and XRD in this paper support the model suggested by Xie Youchang et. al., of close-packed monolayer La_2O_3 dispersed on γ-Al_2O_3 surface, if the amount of La_2O_3 is less than the threshold value (0.28 La_2O_3/100 m~2 γ-Al_2O_3). X-Ray line broaden of γ-Al_2O_3 lattice, due to La_2O_3 highly dispersed on the surface of alumina.