Abstract: The microstructure and mechanical properties of T91 steel tube after aging at 600 ℃ for 0,3 000,5 000,8 000 h were studied by optical microscopy, transmission electron microscopy, tensile testing and impact testing. The results show that the microstructure of T91 steel tube before and after aging at 600 ℃ was tempered martensite and the change of grain size was not obvious. After aging for 3 000,5 000,8 000 h, the tensile strength and yield strength at room temperature had a little changes but the plasticity and toughness reduced. The main precipitates, which were the same with the ones before aging, were still M23C6 and MX after aging for 5 000 h, and a kind of new phase, Laves phase, was found after aging for 8 000 h.