Purpose　-　Super304H　steel　is　a　new　fine-grained　austenitic　heat-resistant　stainless　steel　developed　in　recent　years,　and　it　is　widely　used　in　high　temperature　section　superheater　and　reheater　tubes　of　ultra-supercritical　thermal　power　units＇　boiler.　Currently　intergranular　corrosion　(IGC)　has　occurred　in　a　few　austenitic　stainless　steel　tubes　in　ultra-supercritical　units　and　led　to　boiler　leakage.　The　purpose　of　this　paper　is　to　find　a　nondestructive　method　to　quickly　and　easily　detect　IGC　of　austenitic　stainless　steel　tube.　Design/methodology/approach　-　This　paper　uses　the　nonlinear　characteristics　of　ultrasonic　propagation　in　steel　tube　to　detect　the　IGC　of　Super304H　tube.　Findings　-　The　experimental　results　show　that　the　nonlinear　coefficient　generally　increases　sensitively　with　the　degree　of　IGC;　hence,　the　nonlinear　coefficient　can　be　used　to　assess　IGC　degree　of　tubes,　and　the　nonlinear　coefficient　measurement　method　is　repeatable　for　the　same　tube.　Research　limitations/implications　-　A　theory　of　how　IGC　would　affect　the　ultrasonic　signals　and　lead　to　a　nonlinear　response　needs　further　research.　Practical　implications　-　A　nondestructive　method　to　quickly　and　easily　detect　IGC　is　provided.　Social　implications　-　Using　ultrasonic　nonlinear　coefficient　to　assess　IGC　degree　of　tubes　is　a　new　try.　Originality/value　-　This　paper　provides　a　new　way　to　test　IGC.