Abstract: Changing of microstructure and impact toughness of welded metal of Super 304H steel aged at(600℃-700℃) × 500 h was investigated using optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, X-ray diffractometer and impact test. Results show that microstructure of the welded metal comprises γ phase and precipitates. In the asreceived welded metal, precipitates mainly consists of Nb(C,N) and copper-rich phase and after aging treatment M₂₃ C₆ presents as well. With the increase of aging temperature, amount of precipitates increases accordingly while amount of Nb(C, N) shows a slow increasing trend. The amount of M₂₃ C₆ increases rapidly at the initial stage and becomes stable later. After aging treatment, impact energy of the welded metal decreases obviously. Occurrence of aging brittleness is attributed to the precipitation of M₂₃ C₆ along grain boundaries and it is found that the amount of precipitates is a determinate factor.