Abstract: With 9Cr-2W-3Co martensitic heat resistant steel as the research object, the stress rupture test at 625 ℃ and different stresses (100, 110, 120, 130, 140, 150 MPa) was carried out. The microstructure, precipitated phase characteristics and the stress-rupture limit of the test steel after stress rupture were studied. The results show that comparing with those before stress rupture test, after stress rupture at 100 MPa, the martensitic laths in the matrix of test steel basically disappeared, the dislocation density decreased, the precipitated phase was coarsened, and coarse Laves phase Fe₂(W, Mo) precipitated around Cr₂₃C₆ which distributed along the original austenite grain boundary. The effect of solution strengthening, precipitation strengthening and dislocation strengthening of the test steel was weakened. By fitting the relationship between the loading stress and the stress rupture time, it was deduced that the stress-rupture limit of test steel at 625 ℃ was 93 MPa.