This paper deals with residual stress and welding strength in laser welding of thermoplastics in order to investigate the relationship between them. Thermal stress generated in the workpiece during laser welding process and residual stress remained after the process were observed using photoelastic analysis. Welding strength was estimated by the shear strength. It is found that characteristic photoelastic fringes which reflect the residual stress caused by welding are left at the starting point and end point of the laser irradiated area. It is also found that welding strength increases with order of the characteristic photoelastic fringes which are left at end point of the laser irradiated area.
