Abstract: 304 stainless steel and ferritic stainless steel had good corrosion resistance, which made it difficult to grade the grain size of the material. Using potassium permanganate sulfuric acid solution as corrosion agent, 304 stainless steel was corroded after heating in water bath, and clear and complete austenite grains could be obtained without showing twin boundaries in grains. Using aqua regia placed for about 3 years as the corrosive agent to corrode ferritic stainless steel, clear grain morphology could be obtained, and the accurate evaluation of its grain size level could be realized.