Post weld heat treatment (PWHT) is carried out to improve the capacity of weld structures and to relieve welding residual stress. During the PWHT process, however, a cracking or deformation may occur in a weldment due to the formation of thermal stress and the redistribution of residual stresses. The aim of this study is to analyze the formation of thermal stress during PWHT and then to find the solution which decreases the thermal stress. In regard of the tensile stress, one of the causes of a cracking, the parametric studies are performed in detail through the numerical analysis. Simulations are carried out in the FE software MSC.Marc. PWHT experiments were performed to verify the reliability of the numerical analysis. On the basis of that, the numerical analyses of the thermal stress were carried out for the plate and nozzle shape in many cases such as change of thickness, PWHT temperature history and nozzle shape.