Abstract: Based on the corrosion principle, the austenite grain boundary display method of P91/P92 steels for ultra-supercritical boilers was studied. The results show that the original austenite grain boundary of P91/P92 steels can be clearly displayed by the combined etching method of chemical etching with hot supersaturated picric acid-sodium dodecyl benzene sulfonate-oxygen peroxide aqueous solution and electrolytic etching with 10% (volume fraction) sulfuric acid aqueous solution. The martensite grain size of P91/P92 steels is generally different from the actual austenite grain size by 2-4 grade. For P91/P92 steels, the actual austenite grain size should be tested.