The remarkable feature of electroplated film was grain growth at room temperature and this phenomenon was named 'self-annealing'. The driving force of self-annealing was expected as highly increased defect and surface energy, but the exact mechanism was unknown. In this work, residual stress effect on self-annealing was investigated. During self-annealing, increased temperature accelerated grain growth, and it is concluded that self-annealing is caused by Cu grain boundary diffusion. From the stress measurement by X-ray difractometer (XRD), it was the shown that the average stress release was 500 MPa during self-annealing. The average strain energy is lower than the grain boundary energy, but locally high stress originated from trapped poly ethylene glycol (PEG) whose molecular size is large at the grain boundary, can accelerate grain boundary diffusion of Cu. Therefore, stress is major,driving force of self-annealing, and it plays a role in decomposing PEG molecules and providing sufficient driving force for grain boundary diffusion to Cu atoms near PEG.