In this study, polarization behavior during initial lithiation step was comparatively investigated using a tungsten oxide (WO3) and its reduced one (WO3-x). Ex-situ X-ray diffraction revealed that the overall lithiation process in WO3 was mostly similar to that of WO3-x, which was associated with the typical conversion reaction of transition-metal oxide. In the WO3-x anode, however, insertional lithiation was initiated above 2 V vs. Li/Li+ and a following conversion reaction was observed, which was different charging behavior from WO3 electrode. The severe polarization related with the conversion reaction was alleviated owing to the high electrical conductivity in the mixed-valance state of WO3-x. Furthermore, the different initial polarization in WO3-x anode resulted in the improvement of cycle performance and efficiency.