The oxygen ratio-dependent device performance and reliability of zinc tin oxide (ZTO) thin-film transistors (TFTs) were examined. ZTO TFTs fabricated under pure Ar conditions exhibited a high saturation mobility of 21.5 ${\rm cm}^{2}/{\rm Vs}$ , low subthreshold gate swing of 0.34 V/decade, and high $I_{{\scriptstyle{\rm ON}}/{\scriptstyle{\rm OFF}}}$ ratio $(10^{9})$ , whereas modest mobility of 7.3 ${\rm cm}^{2}/{\rm Vs}$ was obtained for the ZTO TFTs prepared at an oxygen ratio $[{\rm R}={\rm O}_{2}/({\rm Ar}+{\rm O}_{2})]$ of 0.3. The photobias stability $(\Delta V_{\rm th}\approx-2.0~{\rm V})$ of the ZTO TFTs under the Ar only condition was much better than that $(\Delta V_{\rm th}\approx-5.9~{\rm V})$ of the device at $R$ of 0.3, even though their channel layers contained the larger density of oxygen vacancies. This abnormal behavior was attributed to the compressive stress of ZTO films retarding the phototransition of oxygen vacancies.