This study investigates the effects of the grain size of Cu on the bonding performance of Cu-to-Cu bonding. We prepare two Cu films with different grain sizes, fine-grained (FG) and coarse-grained (CG), and bond them using various bonding conditions (temperature and pressure). We next characterize the microstructure of the bonded samples using focused ion beam-scanning electron microscopy (FIB-SEM), unfolding a superior bonding performance for the FG samples: they exhibit active bonding interface migration at 150˚C while the CG samples do not display any noticeable interface migration at temperatures below 300˚C. Further microstructure analysis using electron backscattered diffraction (EBSD) helps us disclose significant differences in grain growth behavior: the FG sample displays abnormal grain growth of some (001) grains, indicative of strain-energy-driven grain growth, whereas the CG sample shows typical curvature-driven grain growth.