We report characteristics of sputtered Ga-doped ZnO films grown in pure Ar (Ar-films) and H2/Ar gas mixture (H2/Ar-films). With increasing water partial pressure (PH2O), the resistivity of Ar-films significantly increased due to the decrease in both free carrier density and Hall mobility. The transmittance in the wavelength region of 300-400 nm for the films also increased with increasingPH2O. However, no significant PH2O dependence of the electrical and optical properties was observed for H2/Ar-films. Secondary ion mass spectrometry analysis revealed that hydrogen concentration in the Ar-films increased with increasing PH2O, indicating that the origin of the incorporated hydrogen is attributed to the residual water vapor in the coating chamber. On the contrary, the hydrogen concentration in H2/Ar-films was almost constant irrespective of PH2O and the concentration is obviously higher than that of Ar-films, indicating that the hydrogen primarily comes from H2 gas and the adsorption species due to H2 gas preferentially adsorb to the growing film surface over residual water vapor. Consequently, the effect of PH2O on the crystal growth was significantly reduced by H2 gas introduction, resulting in stability improvement.