The electrical and optical properties of amorphous indium gallium zinc oxide (a-IGZO) films, which can be used as a channel layer, deposited by radio frequency (rf) magnetron sputtering system at room temperature (RT), were investigated as function of various gas flows. The optical transmittance of films deposited under Ar, O 2 / Ar + O 2 and O 2 / Ar-4% H 2 + O 2 atmospheres in the visible wavelength was consistently above 90% at a wavelength of 550 nm at all gas flows, although the film deposited under Ar-4% H 2 atmosphere exhibited a transmittance of below 50%. The carrier concentration and mobility of the a-IGZO films fabricated under Ar and Ar-4% H 2 were observed slight decrease as a function of the flow, respectively. The thin film transistors (TFTs) with an a-IGZO channel deposited under Ar and Ar-4% H 2 atmosphere exhibited the following good characteristics: V th of 0.34 V, µ FE of 3.6 cm 2 V −1 s −1 , on/off ratio of 10 6 , and S value of 0.04 V decade −1 .