Abstract: Fully dense nanocrystalline Cu with an average grain size of 56 nm was synthesized by a direct-current electrodeposition technique. Tensile tests performed at room temperature indicated that both the strength and the ductility of the nanocrystalline Cu increased by the increment of strain rate, especially a pronounced strain rate dependence of tensile ductility was observed. As the strain rate was raised from 1.04×10^-5s^-1 to 1.04 s^-1, the fracture strain increased from 23.2% to 39.4%, and the ultimate tensile strength increased from 309 MPa to 451 MPa. Two reasons might be responsible for this phenomenon. First, the strain hardening behavior increased with increasing strain rate, resulting in an enhanced uniform elongation. Second, the collective grain-rotations were revealed when the nanocrystalline Cu necked at a higher strain rate, which contributed to the increase of strain after instability.