Both WO3 and NiO are known to be electrochromic materials. In this study, a stack of PET/IZO/WO3/Ta2O5/NiO/IZO electrochromic device (IZO: indium zinc oxide) was fabricated on polymeric substrate (polyethylene terephthalate, PET) by sputtering. Before stacking all layers, each material was separately deposited on a glass substrate under different oxygen flow rates and examined for their microstructures, compositions, optical properties and electrochromic functions. The selected oxygen flow rates have all deposited films to be amorphous except NiO with some crystallites. The electrochromism of WO3 and NiO was materialized by electrochemical insertion of cations (Li+) and electrons into WO3 and NiO films before subjected to potential statics and cyclic voltammetry. The coloration efficiency and transparency of films from these electrochemical tests were evaluated by UV-Vis-NIR spectroscopy. It was found that electrochromism is feasible between +/- 1 V with efficiency reaching 60 cm(2)/C in WO3 and 20 cm(2)/C in NiO, and optical transmittance between 65% - 80% in the range of visible light for films deposited under specific conditions. By analyzing and comparing the coloration efficiency and transparency from electrochemical tests, we can determined a feasible set of process parameters, i.e. oxygen flow rates for sputtering. These parameters then were used to fabricate the stack of PET/IZO/WO3/Ta2O5/NiO/IZO and its electrochromic function was successfully demonstrated.