Sputtering-derived amorphous InGaZnO (a-IGZO) thin films were grown on Si and glass substrates in a mixed ambient of Ar and O₂ at fixed 0.5 Pa working pressure. The influence of O₂/Ar flow ratio on the optical and electrical properties of a-IGZO thin films has been systematically investigated by means of characterization from spectroscopic ellipsometry (SE), X-ray diffraction (XRD), scan electron microscopy (SEM), atomic force microscope (AFM), UV–vis spectroscopy, and electrical measurements. Results have shown that the band gap of the as-deposited IGZO films increases from 3.45 eV to 3.75 eV as the O₂/Ar flow ratio increases from 0% to 20%. Blue shift in band gap and reduction in reactive index with increasing the O₂/Ar flow ratio have been detected. Electrical measurements have indicated the increase in resistivity at higher O₂/Ar gas flow ratio. Related mechanics about the increase in band gap and resistivity have been discussed in detail.