Thin films of Tungsten trioxide (WO3) were deposited on ITO-coated flexible Kapton substrates by plasma-assisted activated reactive evaporation (ARE) technique. The influence of growth and microstructure on optoelectrochromic properties of WO3 thin films was studied. The nanocrystalline WO3 films grown at substrate temperature of 250A degrees C were composed of vertically elongated cone-shaped grains of size 65 nm with relative density of 0.71. These WO3 films demonstrated higher optical transmittance of 85% in the visible region with estimated optical band gap of 3.39 eV and exhibited better optical modulation of 66% and coloration efficiency of 52.8 cm(2)/C at the wavelength of 550 nm.