Electrochromic-based optical modulation devices have attracted much interest because of potential application for electric papers and smart windows. We have reported Ag deposition-based optical modulation device which showed reversible changes of optical states between clear transparent and silver-mirror. In the device, formation of silver halide complexes greatly affected the optical states of silver electrodeposit. In this research, the effects of silver halide complexes were investigated by changing halide anion species in terms of the optical properties of the EC devices. The stability of the silver halide complex induced change in reduction voltage to form silver nanoparticles. Diffusion coefficients and electrode reaction rates also have effects on morphology of deposited Ag nanoparticles and resulting optical properties.