IrO2 thin films were deposited by reactive sputtering in various O2/(O2+Ar) mixing ratios (OMR). The systematic study of the OMR effect on the properties of IrO2 thin films has been reported. It was found that the formation of IrO2 could be classified into two classes, depending on the O2 flow ratio. At low OMR (10-30%), the Ir target and Si substrate were not oxidized and a high deposition rate and high crystallinity IrO2 could be obtained. On the other hand, at high OMR (>30%), the target and Si substrate were oxidized. It resulted in a lower deposition rate of IrO2 and yielded poor structural properties. Moreover, the high OMR provided O atoms, incorporated into the IrO2 thin film. This point could be confirmed by X-ray photoelectron spectroscopy. The excess O defects would also make the resistivity of IrO2 increase as the samples were prepared at high OMR. The effect of substrate temperature on the resistivity was also discussed. It was found that the resistivity of the IrO2 films decreased with an increase of the substrate temperature and a minimum resistivity of 70 μΩcm was obtained as films deposited at 600 °C using 10% OMR.