Abstract: The microstructure of Super304H steel after aging at 650 ℃ for different times was investigated by means of optical microscopy, scanning electron microscopy, X-ray diffractometer and transmission electron microscopy. The results show that the microstructure of Super304H steel after aging for 500 h consisted of austenite phase and precipitate phase, which included Nb(C,N), M23C6 and Cu-rich phase. However, M23C6 was not found in the as-received Super304H steel. The precipitate rate of precipitate phase was relatively fast at the initial stage and became stable after 300 h. With increase of aging time, fine Nb(C,N) particles and Cu-rich phase had not grow obviously. The morphology of M23C6 changed from fine lath-like to chain-like and bulk.