The effects of post‐annealing on amorphous InGaZnO (a‐IGZO) TFTs with ultrathin atomic layer deposited (ALD) AlOxgate insulator (GI) were investigated. The non‐annealed devices exhibited uncontrollable gate leakage current while postannealing in either oxygen (O2) or nitrogen (N2) exhibited noticeable improvements. Optimizing O2‐annealed devices demonstrated the most optimal performance, including a low subthreshold swing of 61.6 mV/dec and a high on/off current ratio over 109. Capacitance‐voltage and X‐ray photoelectron spectroscopy analyses further revealed the underlying mechanism of defect elimination at the AlOx/a‐IGZO interface.