Abstract: The influence of the concentration of KOH and Al(OH)₄^- and current density on the passivation of Al has been investigated using galvanostatic charging method. It was shown that the relation between i and τ^-1/2 obeys the well known non-steady state diffu-sion equation The i～τ^-1/2 curve was composed of two straight lines with different slops and C_Al^S, D_Al have been calculated from the results based on equation(1). The results indicate that when current density is low the passivation of Al electrode is controlled by the diffusion of Al(OH)₄^- ions from the surface into the solution and an anodic passivation film is formed as a result of the precipitation of Al(OH)₃ or Al₂O₃ from the super-saturated Al(OH)₄^- solution near the electrode surface. At higher current densities, the formation of the anodic passivation film is caused by both electro-chemical reaction and the diffussion-controlled precipitation.