We investigated mechanical properties of Cu nanowires with grain boundary by various mechanical deformations using classical molecular dynamics simulation. We simulated the cases of elongation, shearing, rotation, rotated elongation, and compression. Before the first yielding, nanowires preserve the elastic stages, and after this, the mechanical deformation proceeds in alternating quasi-elastic and yielding stages. For Cu nanowires with grain boundary, most of the slip events occurred in the interfaces of grain boundary. For Cu {100} nanowires in this work, all slips occurred in the [110] directions on the {111} planes. In compression case, whole nanowires region, which has originally {100} planes, was transformed into {111} planes during temporary state. However, in other cases, such as elongation, [100] shear, [110] shear, and rotation, only part of nanowires region were transformed into {111} planes.