Optical gaps and electrochromic efficiencies of sputtered tungsten oxide films are studied by focusing attention on the cluster size of the film. The cluster consists of O-W-O network with terminal W=O bonds on its boundary. The quantity of W=O bonds increased with the surface area of the clusters. Raman scattering bands of the O-W-O and W=O are observed between 500 and 1100 cm(-1). These characteristic Raman scattering bands of the film are well reproduced by the combination of four Gaussian shaped Raman bands of the O-W-O and W=O modes. The ratio of integrated Raman scattering intensities (W=O/O-W-O) of these modes are employed as the measure of the cluster size. The cluster is found to increase in size with the elevation of the film deposition temperature. Optical gap was observed to increase inversely proportional to the cluster size of the film. This cluster size dependence appears to have the same origin assigned in nanocrystallites. The electrochemical reduction of the film produces a broad asymmetric polaron absorption band in the optical region from near IR to visible. The coloration efficiency spectrum of this absorption band becomes higher with the cluster size, and its maximum position shifts to a lower energy side. This behavior is explained in terms of a polaron-polaron interaction and a polaron confinement in the cluster.