Abstract: The effects of temper temperature and cooling rate on the impact toughness of a silicon-bearing high chromium martensitic heat resistant steel were studied by a series of Charpy impact test, X-ray diffraction and fracture morphology and microstructure observations. The results show that the retained austenite exists in the microstructure of the steel tempered below 400 ℃, the toughness of the steel decreases greatly when tempered at 450-600 ℃ and the Vicker hardness reaches a maximum, where the steel exhibites a morphology of intergranular plus quasiclevage fractures. The grain boundary weakening caused by the precipitation of chain-like carbides along grain boundaries and the secondary hardening produced by fine dispersed carbides are considered to be the main causes for the temper embrittlement. The strength of the steel matrix decided by the difference in carbides precipitation due to the different cooling rates shows significant effect on the impact toughness of the steel.