A three-dimensional hierarchical WO₃nanostructure with a novel nanosheet-assembled morphology was synthesized through acidification of Na₂WO₄·2H₂O. Firstly, precipitate got from Na₂WO₄·2H₂O and HCl was used as precursor. Secondly, the precursor was pretreated by concentrated acid for 24 h. Thirdly, deionized water was poured into the above solution directly to form the final products. The results revealed that the pretreatment by using high-concentration acid is the crucial factor in order to obtain the flower-like nanostructures. The WO₃samples were characterized by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Brunauer–Emmett–Teller (BET). The results revealed that the samples were flower-like structure. The subunits of the flower-like nanostructures are ultrathin nanosheets with a thickness of about 20 nm. Sensors based on the synthesized WO₃exhibited high response to NO₂. The detection limit can be as low as∼40 ppb level. Such novel WO₃nanostructures may be a promising material for sensor application in detecting NO₂.