Grain growth in Cu-to-Cu bonding was investigated at 250 °C and 300 °C. With highly -oriented nanotwinned Cu (nt-Cu) films, bonding can be achieved with the aid of high diffusivity on Cu (111) surfaces at 250 °C for 1 h. Yet, recrystallization did not occur at this annealing condition. However, recrystallization and anisotropic grain growth took place at 250 °C for 2 h and 300 °C for 0.5 h. The bonding interface was eliminated and extremely large Cu grains grew. The temperature of 250 °C is the lowest temperature ever reported for elimination of the bonding interface. For comparison, the Cu joints fabricated by ordinary Cu films did not have obvious grain growth at 300 °C for 0.5 h.