Abstract: Objective The purpose was to better understand effect of post weld heat treatment process on residual stress of welds in large nuclear power pressure vessels that was made of SA508-III steel. Methods SA508-III steel weldments were prepared, and different post weld heat treatment processes were executed according to ASME standards. The residual stress distribution on the upper and lower surfaces of weldments under states of as-welded and different post weld heat treatment (PWHT) was measured by blind-hole method. At the same time, based on ABAQUS software, numerical simulations were conducted to precipitate stress evolution during welding and PWHT processes of weldments. Effect of creep on residual stress of post weld heat treatment was investigated, and effect of PWHT time on degree of residual stress release was studied. Results The results revealed that creep effect of materials played a very important role in the release of residual stress at 620 ℃ of PWHT. The residual stress distribution obtained by PWHT model considering creep effect was closer to the measured values. The simulation results showed that peak values of transverse residual stress in weld were 200 MPa, 65 MPa, and 50 MPa respectively under 1 h, 6 h, and 40 h of PWHT time. Conclusion With the extension of PWHT time, stress level of weld kept decreasing, and significant change occurred within 6 h of PWHT, which provided a data basis for the optimization of PWHT process.