In this paper, a MoO3 thin film was firstly electrodeposited on flexible ITO/PET substrates. Its electrochromic properties were examined using an electrochemical workstation coupled in situ with a charge-coupled device (CCD) spectrophotometer. The film was characterized by XRD and AFM. The electrodeposited MoO3 thin film on the ITO/PET substrate showed a large transmittance contrast between the colored and bleached states, a quick response, and good durability and reversibility. The low working potential range, -0.5 to 1.6 V (vs. Ag/AgCl), for the MoO3/ITO/PET film prepared in this work makes it a good candidate for flexible smart windows, as well as other plastic electrochromic devices.