The effect of the heat treatment on electrochromic properties of $ extrm{MoO}_3$ thin films is investigated by studing optical modulation, optical density, response time, and cyclic voltammetry. From the results of XRD analysis, heat-treated at $450^{circ}C$ in air for 1 hour Moo3 thin films are found to be crystalline while as-deposited and heat- treated at low temperature ($leq;300^{circ}C$) film are amorphous. The electrochromic devices using as-deposited $ extrm{MoO}_3$ films exhibit good electrochromic properties compared to those using the heat treated $ extrm{MoO}_3$ films. It has shown that the heat-treatment affected the reversible color change and the electrochromic properties of $ extrm{MoO}_3$ films.