Abstract: The accelerated creep tests of P91 steel at 650 ℃/70 MPa were carried out.The microstructure and hardness changes during creep were characterized by Brinell hardness tester, SEM, TEM and EDS.The effects mechanism of microstructure evolution on the hardness reduction were analyzed.The results show that the hardness of P91 steel decreases slowly in the early stage of creep(less than 40%creep life), and the decreasing range increases significantly in the middle and late stage of creep.The hardness decreases from 222 HB to 175 HB after creep for 2935 h.The mechanism of microstructure degradation during creep is that the dislocation density decreases significantly, the grain size increases obviously, M₂₃C₆ aggregates and grows, and the lath morphology of martensite gradually loses.The decrease of dislocation strengthening is the main reason for the hardness decrease of P91 steel.