Whether low strain hardening is an intrinsic behavior of nc metals is still an open question. To make clear this issue, in this paper repeated stress relaxation experiments were performed on a nanocrystalline and coarse-grained copper, respectively. The mobile dislocation density during plastic strain, which directly determines the performance of strain hardening, was deduced from this experiment. The mobile dislocation density of nanocrystalline copper decreased much slower than coarse-grained copper. The high mobility of the dislocation in nanocrystalline copper indicate the dislocation seldom interact with each other and generate sessile barrier, and render the nanocrystalline copper a more diminutive strain-hardening capacity than coarse-grained copper.