Intergranular　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.