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Copper nitrate and sodium tungstate were employed to prepare nano-sized cupric tungstate (Ⅵ) dihydrate (CuWO4·2H2O) by hydrothermal method. Formation processes of Cu2WO4(OH)2 precursors were discussed, and the synthesis mechanism of CuWO4·2H2O precipitates from Cu2WO4(OH)2 precursors during the process of hydrothermal reaction was established. Morphological and microstructural investigations of precursors and precipitates were carried out using high resolution transmission electron microscopy (HRTEM) and X-ray diffractometer (XRD). Results show that the synthetic products CuWO4·2H2O particles are mostly spherical in the size range from 20 to 30 nm. Transmission electron microscopy (TEM) observations indicate that the formation of CuWO4·2H2O follows in situ crystallization mechanism in the process of hydrothermal reaction. In the long term hydrothermal environment of high pressure and temperature, Cu2WO4(OH)2 precursors are surrounded by WO42- ions, which are adsorbed and diffused on the surface of Cu2WO4(OH)2. By dehydrations and molecular rearrangement, CuWO4·2H2O precipitates heterogeneously nucleate here. WO42- ions react with Cu2WO4(OH)2 through CuWO4·2H2O layer until WO42- ions have been depleted.