Amorphous indium gallium zinc oxide (a-IGZO) films were grown and used to detect ozone gas. When the deposition temperature was below 400°C, the IGZO surfaces were uniform and smooth. The most stable gas sensor made from an IZGO film was deposited at room temperature and annealed at 200°C for 30min under pure argon gas at 200mTorr. Films annealed at 200°C are much more sensitive to ozone gas than those annealed at other temperatures (the sensitivity was enhanced from 0.45 to 15). The desorption time of the ozone improved from 2541 to 327s. Annealed IGZO films can detect ozone at 35–168ppb with resolution of about 20ppb. These films detected ozone at ppb level, which means that IGZO has the potential to serve as the active material in gas sensors.