In this article, the diffusion behavior between 6061 aluminum alloy substrate and Ni–5wt.%Al coating was studied by changing the heat treatment temperature and processing time. The experiment results of EDS and XRD showed that a diffusion zone generated between coating and substrate even if there was no change in phase compositions of the coating. The thickness of diffusion zone increased with the process temperature accompanied with the scale-up of Al concentration. In the decrease direction of aluminum concentration, the results of computation indicated that the phase composition transformed into NiAl3, Ni2Al3 and Ni solid solution in the diffusion zone. It was concluded that microstructure and phase composition in diffusion zone between 6061 aluminum substrate and Ni–5wt.%Al coating was mainly controlled by aluminum diffusion during different heating conditions.