The processes of initial plastic deformation of single crystal Cu in nanoindentation are simulated by using quasicontinuum method. The indenters with three different widths are used and the corresponding curves of load-displacement are analyzed to determine the critical loads of dislocation emission, which are basically in agreement with the loads predicted from energy point of view. The mechanical and microstructural characteristics of dislocation nucleation are revealed by analyzing the displacement field beneath the indenters. The general regularity of the increase of geometrically necessary dislocations beneath the indenter is presented. The motions and annihilations of dislocations in the unloading are discussed qualitatively.