The phase transformations in tungsten oxide (WO3) thin films have been investigated in the temperature range of 30-500 °C using transmission electron microscopy (TEM) and selected area electron diffraction (SAED) measurements. The results indicate that the phase transitions in WO3 thin films occur in sequence exhibiting various crystal phases above room temperature in the order: monoclinic orthorhombic hexagonal. The crystallographic distortions and tilting of the WO6 octahedra are responsible for the phase transitions and significantly affect the electronic properties and, hence, the device applications of WO3.