The sintering behaviors of the nickel model catalysts supported on γ-alumina were studied under wet hydrogen atmosphere as a function of time and temperature. Two types of initial crystallite size distribution, i.e., the aggregated and the uniformly dispersed, can be obtained by controlling the sputtering conditions. The same region of a specimen was examined with a transmission electron microscope after each heat treatment to follow the change of each nickel crystallite. The surface of nickel crystallite was found in the oxide state as observed by the electron spectroscope for chemical analysis. The various events which had occurred during the first one hour were identified, and they can be explained by applying all the possible sintering mechanisms, i.e., the crystallite migration, the atomic migration and the combination of the two.