Tungsten trioxide (WO3) has attracted great attention due to its promising and wonderful properties. In this study, the influences of calcined temperature on the structural, morphological and optical properties of WO3 were investigated. Sodium tungstate dehydrates (Na2WO4.2H2O, 99.9%) powder was used as starting precursor. A sol–gel spin coating technique was used to synthesize pure WO3 thin films. The as- deposited films have been annealed at 300 oC, 400 oC, 500 oC, 550 oC and 600 oC for 2 hours. The structural properties of thin film samples have been studied by the X-ray diffractometer (XRD) with Cu Kα radiation of wavelength 1.542 Å. X- ray diffraction data has been used to find out changes in the crystallite size and to determine the phases present in the films. Field emission- scanning electron microscopy (FE-SEM) has been performed to investigate the surface morphology of the prepared samples. The optical band gap energy of the thin films has been investigated and analyzed using UV-Vis spectrophotometer equipped with an integrating sphere and a Spectral reflectance standard. The absorbance of the films was measured in a wavelength range of 190–2500 nm. From the results it was found that the characterized peaks were associated to the crystalline planes of the monoclinic phase of WO3. The crystallite sizes increased with increasing calcination temperatures. It was observed that the WO3 had a sphere-like structure composed of numerous nanoparticles. The absorbance spectra show that the there is a red shift in the absorption edge as the calcined temperature increases The optical energy gap was decreased as the calcined temperature increased. The estimated energy gap values for the thin film sample under consideration was about 2.95, 2.58, and 2.023 for sampled calcined at 400 0C 500 0C, and 550 0C respectively.