Transparent amorphous Li2.5TaOx thin films with different thicknesses were deposited by RF magnetron sputtering. The ionic conductivities of as-deposited films determined by ac impedance spectroscopy were of the order of 10(-8) S/cm at room temperature. The surface roughness increased rapidly with the film thickness increasing as confirmed by scanning electron microscopy and atomic force microscopy. Inorganic monolithic electrochromic devices (ECDs) based on WO3//NiO complementary structure were fabricated using the Li2.5TaOx with different thicknesses as the ion conductor layers. The electro-optical performance of the ECDs was characterized by cyclic voltammetry and chronoamperometry. The ECDs showed a highest optical modulation of 40% at 550 nm driven by -1.5 V (coloration) and 1 V (bleaching) with switching time of 30 s. Both the amount of transferred charge and coloration efficiency were found to be highly dependent on the film thickness of Li2.5TaOx. layer. The high charge transfer resistance at the Li2.5TaOx/WO3 interface and poor growth of WO3 layer due to the high surface roughness of Li2.5TaOx may well account for the decreased device performance.