Abstract: The invention relates to a method of forming aluminum diffusion layer including the steps of dipping a ferrous base alloy workpiece containing nickel of more than 8.0% by weight in a molten metal bath of aluminum or aluminum alloy having temperature of 650.degree. to 750.degree. C. for 30 to 120 seconds, subjecting the ferrous base alloy workpiece thus treated to a first heat treatment at temperature from 750.degree. to 850.degree. C. for at least 60 minutes, subsequently subjecting the ferrous base alloy workpiece thus treated to a second heat treatment at temperature of 900.degree. to 1000.degree. C. for at least 30 minutes, and further subjecting the ferrous base alloy workpiece thus treated to a third heat treatment under predetermined conditions at set temperatures and durations for the separation of an aluminum compound layer formed on the workpiece and for the formation only of aluminum diffusion layer on the surface of the workpiece.