We established a method for preparation of iridium oxide thin film by the sol-gel dip-coating process where iridium chloride was used as a starting material. The coating solution was prepared by reacting iridium chloride, ethanol and acetic acid. Iridium oxide coating was formed at 2.0 cm/min withdrawing rate. The coating films heat treated at 300 degrees C did not contain impurities. Iridium oxide crystallized at temperatures above 450 degrees C. Both crystalline and amorphous iridium oxide thin films showed electrochromism. The change in transmittance of the crystalline Ir2O3 film is larger than that of the amorphous Ir2O3 under the same experimental conditions. The transmittance of the crystalline thin firm (film thickness 200 nm, measured at 400 nm) decreased 13.0% on application of 3 V for 1 s.