Microstructure of Al-Si coated PWA 1426 alloy was studied. Diffusion coated specimens were heat treated for 4hr at $870{sim}1087^{circ}C$ and then were examined the changes of microstructure and interfacial compound by optical microscopy, SEM and EDS. According to the result of EDS, it is supposed that the coated layer was composed of $Ni_2Al_3$. When diffusion treatment was conducted at $1087^{circ}C$, coated layer varied from $Ni_2Al_3$ to NiAl phase and composed of mixed, denuded and inter-diffusion layer.