Formation of well-aligned and single-crystalline ZnGa2O4 nanowires on sapphire (0001) substrates has been achieved via annealing, of the Ga2O3/ZnO core-shell nanowires. Ga2O3/ZnO core-shell nanowires were prepared using a two-step method. The thickness of the original ZnO shell and the thermal budget of the annealing process play crucial roles for preparing single-crystal line ZnGa2O4 nanowires. Structural analyses of the annealed nanowires reveal the existence of an epitaxial relationship between ZnGa2O4 and Ga2O3 phases during the solid-state reaction. A strong CL emission band centered at 360 nm and a small tail at 680 nm are obtained at room temperature from the single-crystalline ZnGa2O4 nanowires.