Owing to the indirect band gap nature, Ge exhibits poor optical properties, limiting its usage for optical devices. However, since the direct band gap of Ge is only higher by 0.14 eV than the indirect band gap, band gap engineering has drawn much attention to realize the direct band gap. Here, we report a strategy to design the direct band gap in Ge/Sn coreshell nanowires (NWs), based on first-principles calculations. For -oriented NWs, we show that the direct band gaps can be tuned by controlling the diameter and the core-to-shell ratio. We find that the intrinsic strain induced by the lattice mismatch between Ge and Sn drives an indirect-to-direct band gap transition. Even for Ge/Sn coreshell NWs with intrinsically indirect band gaps, the direct band gaps can be achieved by applying an external tensile strain lower than the critical values for pure Ge NWs and bulk Ge. The optical transitions of the direct band gaps are all dipole-allowed, suggesting that -oriented Ge/Sn coreshell NWs are promising for applications as light emitters.