Abstract: Local postweld heat treatment (local PWHT) is usually performed for tempering and relaxation of residual stresses. Many factors have an influence on PWHT procedures and the heated width is the most important parameter controlling the effectiveness of local PWHT. However,the determination rules of this parameter are very different in the different codes. In this study,a direct method to assess the effectiveness of local PWHT was used based on the thermal-visco-elastic-plastic FEM with the consideration of creep phenomena. This method clearly shows the whole history of stress relief during PWHT and a critical heated width can be obtained. The pipe with original welding residual stresses is analyzed under different conditions of PWHT. Investigations show that the maximum residual stress after PWHT decreases when the heated width increases. When the heated width becomes large enough,the residual stress after PWHT changes very slowly and a critical heated width can be found which gives a residual stress close to the value obtained from th uniform PWHT. A series of different PWHT conditions are studied to find the critical heated width by using this method. It is found that a heated area of 5 √rq centered on weld seems resonable from the view of stress relief.