With an atomic layer deposition (ALD) method, mixed oxide thin films can be formed. This feature allows to experimentally deduce the mixing ratio of oxide components, which yields optimal device properties. Herein, all‐oxide bottom‐gate thin‐film transistors (TFTs) having binary ZnOx–InOy channel are fabricated. To find composition, which yields energy‐efficient TFTs of low subthreshold swing (SS) characteristics, mixed oxide channels of ZnOx and InOy are prepared, using a plasma‐enhanced ALD method and liquid precursors of diethyl zinc and 3‐(dimethylamino)propyl‐dimethyl indium. Channel comprised of ZnOx:InOy = 1:2 exhibits the best transfer characteristics with a highest field effect mobility and a lowest SS of 30.3 cm2 V s−1 and 0.14 V dec−1, respectively. The variation on SS is discussed quantitatively in terms of channel depletion and the quantitative description on oxygen vacancy.