We demonstrate the fabrication of WO3 nanoflakes by dealloying with subsequent thermal oxidation. Sputtered amorphous W0.68Cu0.32 alloy films were dealloyed in HNO3 solution where Cu was selectively removed and W self-assembled into crosslinked cubic phase W nanoflakes. This kind of W nanoflakes was transformed to monoclinic WO3 nanoflakes by thermal oxidation at 500 degrees C. Gas sensor made using the WO3 nanoflakes showed promising NO2-sensing performance.