A series of WO₃ samples with different crystalline phases were prepared by the thermal decomposition method from ammonium tungstate hydrate. X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy, and N₂ adsorption-desorption were used to characterize the crystalline phase, morphology, particle size, chemical composition, and surface area of the WO₃ samples. The formation of hexagonal (h-WO₃) and monoclinic (m-WO₃) crystal structures of WO₃ at different temperatures or different times was confirmed by XRD. m-WO₃ is formed at 600 °C, while m-WO₃ starts to transform into h-WO₃ at 800 °C. However, h-WO₃, which forms at 800 °C, may transform into m-WO₃ by increasing the calcination temperature to 1000 °C. SEM results indicate that m-WO₃ particles exhibit a bulky shape with heavy aggregates, while h-WO₃ particles exhibit a rod-like shape. Moreover, m-WO₃ crystals are sporadically patched on the surface of the h-WO₃ rod-like particles, resulting in the exposure of both m-WO₃ and h-WO₃ on the surface. It is observed that the monoclinic phase (m-WO₃)/hexagonal phase (h-WO₃) junction was fabricated by tuning the calcination temperature and calcination time. The relative ratios between m-WO₃ and h-WO₃ in the phase junction can readily be tailored by control of the calcination time. The photocatalytic activities of WO₃ with different crystalline phases were evaluated by the photocatalytic degradation of rhodamine B as a model pollutant. A higher photocatalytic activity was observed in the WO₃ sample with the m-WO₃/h-WO₃ junction as compared with the sample with only m-WO₃. The improvement of photocatalytic activity can be attributed to the reduction of the electron-hole recombination rate owing to the formation of the phase junction, whose presence has been confirmed by HRTEM and photoluminescence spectra.