Example embodiments provide an electrochromic device using polyphthalate as an electrochromic material and a process for preparing the same. The electrochromic device in accordance with example embodiments enables the production of a solid nanoscale electrochromic device having a bistability using polyphthalate as an electrochromic material, and easy formation of a red electrochromic layer which was not reported in a conventional art. Accordingly, the electrochromic device of example embodiments can be usefully employed for fabrication of RGB color electrochromic displays.