Abstract: The welding thermal cycles with different peak temperatures for T92 steel were measured with the Gleeble-3800 thermal simulation test machine, and then the specimens were tested with charpy notch in room temperature. Microstructures and its charpy impact fracture morphologies of the thermal simulation specimens were observed using optical microscopy and SEM. The results show that micro-structural brittleness can occurr easily for austenite crystal grain when welding temperature higher than 900℃ and quasi-cleavage fracture displays clearly. While the excellent ambient temperature impact toughness could be maintained when welding temperature below 900℃ with the fine uniform ductile fracture displaying. The micro-structure brittleness was resulted from high content of strong carbide and nitrogen formation elements which solid dissolved in austenite under high temperature and then the diffusion speed being slower than the speed of grain boundary migration, thus formed room temperature over-saturation micro-structure.