Abstract: The P91 weld joint used in the experiment is re-tempered according to the heat treatment process. The effect of tempering times on its mechanical properties, including that at room temperature and high temperature, was then investigated in relation to the evolution of its microstructure using optical microscopy (OM), X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and tensile tests. Results indicate that the strength at room and high temperature of the heat affected zone increases first and then decreases with an increase in tempering times. In addition, the microstructure is still tempered sorbite that reserves the orientation of lath martensite, and the main phases are the α-Fe phase and Fe-Cr phase. After being tempered once, its high-temperature tensile strength reaches a maximum of 232.66 MPa. These improved mechanical properties relate to the dispersed distribution of niobium and vanadium carbonitride precipitate particles, dislocation tangles, and dense metastable dislocation networks. With the increase of tempering times, the distribution of Cr₂₃C₆ precipitates gradually undergoes a change from being isolated to displaying a continuous manner in the grain boundaries; this can embrittle the grain boundary and reduce the strength while improving toughness.