Various ion storage layers were fabricated on ITO-glass substrates by a dry deposition process to study their effects in electrochromic devices. Dense nickel oxide (NiO) film was formed using micro-sized NiO particles, a porous film was formed using nano-sized NiO particles, and a porous antimony tin oxide (ATO) film was formed using nano-sized ATO particles. Electrochemical analyses revealed that the nano-porous NiO layer had a high charge capacity with a low charge transfer resistance. Moreover, an electrochromic device using a NiO film with nano-sized pores had an optical transmittance difference of 42% and a stable cyclic transmittance for 1 h at a wavelength of 630 nm. We assessed the effects of the different ion storage layers by evaluating the electrochromic device in terms of the following important properties: 1) high charge capacity, 2) low charge transfer resistance at the interface between film and electrolyte, and 3) high diffusion rate from film to electrolyte. Based on these criteria, we found that the ion storage layer formed with nano-sized NiO particles best satisfied these conditions. Finally, we confirmed that stable and high electrochromic performance can be achieved through improving these properties in ion storage layers.