Hydrogen storage alloys can absorb and desorb a large amount of hydrogen per unit volume at room temperature and pressure of the atmosphere. Hydrogen storage alloys can be applied to industrial applications and those have considerable potential, such as for hydrogen storage tank, chemical compressors and chemical heat pumps. In case that hydrogen is formed by water electrolysis, hydrogen contains saturated water vapor and if hydrogen storage alloys absorbs hydrogen contaminated with water vapor, the rate of hydrogen absorption / desorption decreases significantly. The ability of industrial applications of hydrogen storage alloys decreases with decreasing the rate of hydrogen absorption / desorption. In this study, hydrogen desorption behavior of hydrogen storage MmNi5-based alloy degraded by water vapor is investigated numerically and experimentally. The single reaction of hydrogen and the surface layer of alloys degraded by water vapor is treated in details and the model in hydrogen desorption process is proposed by examining some mathematical modeling and calculation. As a result, experimental data are in good agreement with calculated ones. It can be estimated that the desorption process in degraded hydrogen storage alloys is controlled by hydrogen diffusion step in oxide layer.