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A facile method to synthesize WO 3 ·H 2 O square nanoplates via a mild aging (50°C) of ion-exchanged precursor was developed. The ion-exchanged precursor was prepared by passing the sodium tungstate solution (Na 2 WO 4 ) through a protonated cationexchange resin, and used as impurity-free acidified solution (H 2 WO 4 ) for synthesizing WO 3 ·H 2 O nanoplates. No shape-directing additive was employed. It was observed that the yellow particles were precipitated under aging at 50°C. After aging for 8 h, the precipitated particles were characterized as the WO 3 ·H 2 O nanoplates, and their morphological evolution to square platelet proceeded with an increase of aging time. After aging for 24 h, the WO 3 ·H 2 O square nanoplates were predominantly synthesized. The square nanoplates consisted of a few or several stacked thin layers (thickness, ³10 nm/layer), and provided the well-defined {010} facet for two dominantly exposed surfaces and {101} side facets. Their lateral dimension reached several hundreds of nanometers. It is thus demonstrated that the mild aging (50°C) of the ion-exchanged precursor is a simple and impurity-free synthetic route for WO 3 ·H 2 O square nanoplates. In addition, the monoclinic WO 3 nanoplates were successfully obtained by dehydration-induced topochemical transformation of the WO 3 ·H 2 O nanoplates.