Abstract: Microstructure and toughness changes of Super304H steel aged at 700℃ was investigated using optical microscopy, scanning electron microscopy, X-ray diffraction and impact tester. Results show that M23C6 carbide was precipitated during aging treatment. The amount of M23C6 was increased rapidly at the initial stage and then became stable. M23C6 was precipitated along grain boundary in a chain-like manner, and with the increase of time its size increased rapidly. Aging brittleness of Super304H steel was attributed to the precipitation of M23C6 along grain boundary, and the brittleness tended to be severe for the steel containing higher amount of M23C6.