The electrochromic (EC) performance of a WO3 film fabricated using a kinetic spray technique (nanoparticle deposition system, NPDS) was examined as a function of the particle size and phase of the WO3 particles. The total transmittance change of the deposited WO3 film containing crystalline nano-sized WO3 particles was 56% at 800 nm, which resulted in a large shift in its transmission spectrum after coloring. Moreover, the charge density of the EC film using crystalline nano-sized WO3 particles was 22.06 mC cm(-2), which was 2.8 times higher than that of the film composed of crystalline micro-sized WO3 particles (7.97 mC cm(-2)). Using crystalline nano-sized WO3 particles greatly increased the surface area, increased the number of absorption sites for Li ions inside the film, and thereby improved the charge density and transmittance. The transmittance change of the EC films using dual-phased nano-sized WO3 particles was 64% (at 800 nm), which was still higher than that with crystalline nano-sized WO3 particles. The amorphous phase in the dual-phase nano-sized WO3 film provided good diffusion of Li ions; it had a high diffusion coefficient of 1.53 x 10(-12) cm(2) s(-1) that derived from its loosely packed atomic structure. The amorphous region in a heterostructured WO3 film was effective for the fast diffusion of Li ions, while the crystalline nano-sized region in such a film provided a large surface area for an increased number of absorption sites for Li ions. This resulted in a dramatic improvement of the EC performance.