The aim of the present work is a synthesis and investigation of the structural properties of tungsten trioxide (WO3) nanoparticles. The prepared samples were calcined at 400 and 450˚C. The structural, compositional, morphology and molecular properties of the samples were investigated by using X-Ray Diffraction, Scanning Electron Microscopy, Energy-Dispersive X-Ray Spectroscopy and Fourier Transform Infrared Spectroscopy, respectively. The structural parameters of the samples were studied by the Size-Strain Plot (SSP) method. The results show that the particle size increases with increasing calcination temperature and the amount of impurities also decreases. The photocatalytic activity of the tungsten trioxide nanoparticles was evaluated by the photocatalytic discoloration of methylene blue in aqueous solution.