In the past decade, core/shell nanowires (NWs) have attracted much attention due to the broad variety of potential applications of these structures in future nanoelectronic and nanophotonic devices. Because of the lattice mismatch between the core and shell materials, crystal dislocations often form to relax the mismatch strains. In this article, we propose a relaxation mechanism for the misfit strains generated in the core/shell NWs, in which lattice dislocations nucleate from the outer surfaces and then propagate to the core/shell interface. An analytical model is developed to predict the critical shell thickness corresponding to defect-free core/shell NWs with respect to the growth direction.