Crystal defects induced by irradiation obstruct dislocation movement. Hence, the critical resolved shear stress of irradiated material increases. While part of the radiation defects are swept by dislocations released from dislocation sources; therefore localization of plastic deformation are caused by a decrease of radiation defects at the partial region. In this study, in order to predict increase of flow stress due to irradiation, information of density of radiation defects is introduced into a hardening modulus of crystal plasticity. Moreover, a decrease of the work-hardening ratio is represented by considering the disappearance of the radiation defects originating in the dislocation movement. Values of the controlling parameters operating effect of the radiation defects on the flow stress are estimated by a molecular dynamics simulation. We conduct crystal plasticity simulations for copper single crystals under a simple tensile condition. The macroscopic stress-strain responses such as the increase of yield stress and the decrease of work-hardening ratio due to irradiation are numerically predicted. We investigate the effect of radiation defects and the dislocation behaviors on processes of the strain localization.