Electrochromic cells were fabricated via a nanoparticle deposition system (NPDS) using different particle sizes of monoclinic tungsten oxide (WO3). Mixtures of micro- and nano-sized WO3 powders in the ratios of WO3 (micro):WO3 (nano) = 9:1, 5 : 5 and 1 : 9 vol%, were used in this study. NPDS, which was used to fabricate the electrochromic layer, is a low-cost process that can cover a large deposition area and provides a highly porous film. This method can replace sol-gel and sputtering methods, which are expensive and have environmental issues. The WO3 electrochromic layers displayed different surface structures that could adsorb Li+ ions. The transmittance change, cyclic switching speed and coloration efficiency (CE) results demonstrated that the electrochromic cell made with the mixed WO3 (micro):WO3 (nano) powders had better performance than that of the electrochromic cell made with separate micro-sized single powders. Various analyses showed that the WO3 mixed powders contained larger sites for Li+ ion adsorption compared with the single-sized powder because of a structure consisting of a compact layer of micro-WO3 with a porous layer of nano-WO3. Consequently, a cell composed of mixed-particle electrochromic layer showed higher transmittance change, CE and electrochromic performance than a cell made with a micro-sized single powder.