Diffusion behaviors of aluminum (Al) in zinc-oxide (ZnO) single crystals were measured by means of ion implantation technique and SIMS depth profiling. It was found that Al concentration profile had a peculiar shape with a constant-concentration region and a steep tail, which was also found in profiles of other donors such as Ga and In. Detailed analysis of the profiles revealed that the diffusivity of Al was proportional to the square of Al concentration and its intrinsic diffusivity was much smaller than previously reported values. Oxygen diffusion experiments were also performed and the implantation of Al ions enhanced the oxygen diffusion coefficients by about 20 times. This result indicates that oxygen interstitial diffusion occurs in n-type ZnO.