The authors studied the properties of aluminum diffusion coated layers on stainless steels: SUS 430, SUS 304, and SUS 321. The results obtained were as follows. (1) The aluminum diffusion coated layer consisted of an additive layer and a diffusion layer. The additive layer having an almost uniform aluminum concentration, was composed of Fe₃Al and a solid solution of component elements; it was observed that it had a structure consisting of columnar and ununiform grains. In addition, the additive layer of SUS 321 (about Hv 800) was much harder than those of SUS 430 (about Hv 400) and SUS 304 (about Hv 500). (2) The variation in aluminum concentration of diffusion layer was continuous in SUS 430, but those in SUS 304 and SUS 321 were discontinuous. The structure of diffusion layer consisted of gross grains in each kind of steels. (3) Tensile stress appeared in the additive layer of each aluminum diffusion coated stainless steel. The decreasing order of the tensile stress was arranged as follows: SUS 430>SUS 304>SUS 321. (4) There was a tendency of cracks on the surface as follows: The size of crack was proportional to the tensile stress and the thickness of additive layer. (5) The amount of foreign matter adhered to the spots of high aluminum concentration on the surface increased with the rise of temperature for diffusion coating.