ZnO nanowires were synthesized by the heat treatment of ZnS substrates with Ga droplets in the air, and their morphological and structural properties were then investigated. The nano-sized ZnO wires were formed on thin ZnO layers accompanied with the formation of ZnGa2 O4. It was found that the growth direction of the nanowires is ZnO[0001], and which is parallel to ZnGa2O4[010]. The growth mechanism of the ZnO nanowires is discussed based on the thermal stability or enthalpy of formation of the oxides. © 2009 The Surface Science Society of Japan.