Abstract: Imapact tests were performed after T91 steel tube high temperature reheater serving for 0 h, 60 000 h and 115 000 h, and then the ductile-brittle transition temperature was determined. The toughness variation of T91 steel after long-time serviing was studied by combining fracture and impact characteristics. The results show that the ductile-brittle transition temperature of the T91 steel was -40 ℃ after it serving for 115 000 h. The T91 steels tube still owned good plasticity and ductility after service for a long time at high temperatre, although their cleavage fracture rate increasing and its crack propagation decreasing. After T91 steels serving for 60 000, 115 000 h, their crack propagation energy had no larger difference at the same impact tempact temperature, which was associated with microstructure ageing disiocation strengthening.