Abstract The newly developed Super304H austenitic stainless steel is being widely used in ultra supercritical fossil fired boilers because of its superior high temperature strength, good oxidation resistance and steam corrosion resistance. But its high carbon content increases greatly its intergranular corrosion susceptibility. A 1000 MW ultra supercritical fossil fired boiler being installed in South China experienced a severe superheater leakage accident recently during a water pressure testing. After failure analysis, it was found that the superheater tubes, made of Super304H, were attacked by stress corrosion cracking which was induced by intergranular corrosion. The external reason for the intergranular corrosion was the bare laying of the tubes at seaside environment for a few months and the internal reason was the very high intergranular corrosion susceptibility of the tubes. This aroused our concern on the intergranular corrosion resistance of this material and further experiment was carried out by selecting different batches of Super304H tubes to evaluate their intergranular corrosion susceptibility. The conclusion was that the high intergranular corrosion susceptibility of the Super304H tubes might be induced by the unqualified match of carbon and niobium or the improper delivery heat treatment technology. It is therefore strongly recommended that more attention be paid to this novel material regarding its intergranular corrosion susceptibility and the chemical composition and delivery state heat treatment technology for standard specification of this material should be further investigated and optimized.