A facile hydrothermal process has been employed to synthesize WO3 nanocubes using urea as an assistant agent. The crystal structure and morphology of WO3 nanocubes have been characterized by X-ray diffraction (XRD), scanning electron microcopy (SEM) and transmission electron microcopy (TEM). XRD, SEM and TEM images of the products illustrate that WO3 square nanostructures have a single orthorhombic phase, length of ~ 100-200 nm and thickness of ~ 40 nm. The ethanol sensing property of as-prepared tungsten oxide nanocubes is studied at ethanol concentration of 10-1000 ppm and working temperature of 160-400 °C. The result shows that the WO3 nanocubes have high sensitivity to ethanol because of their small size and special morphology.