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We scrutinized the barrier capability of SiOx, plasma Al2O3 (P-Al2O3)/SiOx, and SiNx/SiOx passivation layers (PLs) on the environmental stabilities of back-channel etched Al-doped InSnZnO (Al-ITZO) TFTs at 85 °C with a relative humidity of 85 % for 30 days. Turn-on voltage (VON) of SiNx/SiOx-passivated TFTs was dramatically shifted to the negative direction and became conductive. Compared to those of SiOx and P-Al2O3/SiOx films, more hydroxyl groups existed at the PL/active interface of SiNx/SiOx-passivated Al-ITZO films. Water vapor transmission rates showed that abnormal behavior was not attributed to barrier capability of PL against the water vapor. When all TFTs were kept at 85 °C for 30 days in an air-drying oven, only the VON of SiNx/SiOx-passivated TFTs shifted negative direction and finally became conductive. Secondary ion mass spectroscopy (SIMS) results revealed that this abnormal behavior originates from the formation of oxygen vacancy due to highly existed hydroxyl group at SiOx/Active interface at an elevated temperature.