ABSTRACTIntergranular corrosion (IGC) is one of the major localized corrosion problems faced by austenitic stainless steel components in ultra-supercritical power plants, which occurs along the grain boundaries of material and results in loss of engineering properties. Ultrasonic technique was applied to the early detection of IGC in austenitic stainless steel super304H. Acoustic attenuation and nonlinear parameter were measured at multiple excitation frequencies, and an equivalent attenuation factor and an equivalent nonlinear factor were defined to evaluate the IGC damage. Results indicated that both factors increased monotonously as the degree of IGC developed, and the equivalent nonlinear factor is more sensitive to IGC at early stage. Thus they can be utilized for early detection of IGC in austenitic stainless steel.