Abstract The electrical performances of amorphous InGaZnO (a-IGZO) thin film transistors (TFTs) deposited at various nitrogen/oxygen partial pressure ratios are investigated under light illumination with different wavelengths. It is found that a-IGZO TFTs show non-negligible sensitivity to light illumination with wavelength below 600 nm (∼2.0 eV), related to the photo-excited ionization process of neutral-oxygen-vacancy (Vo) in a-IGZO. With a moderate level of nitrogen doping in the a-IGZO active layer, the concentration of Vo shows an apparent decrease, as revealed in x-ray photoelectron spectroscopy analyses. Correspondingly, the device exhibits considerably improved electrical stability after light illumination, presenting dramatically reduced threshold voltage and sub-threshold swing shifts. Therefore, nitrogen doping in a-IGZO active layer is an effective approach to enhancing the stability of a-IGZO TFTs against light illumination, and should be ascribed to the decreased sub-gap density of states originated from Vo near the valence band maximum in a-IGZO active layer.