There are few reports that evaluated quantitatively the effect of the PWHT (Post Weld Heat Treatment) on the weld residual stress relaxation. We studied the residual stress for butt welding of thick plate. We conducted the experiment and the thermo-elastic-plastic finite element analysis. We became to be able to estimate the residual stress distribution of the butt weld. Also we clarified the effect of the PWHT on the weld residual stress.