We investigated the impact of photon irradiation on the stability of gallium-indium-zinc oxide (GIZO) thin film transistors. The application of light on the negative bias temperature stress NBTS accelerated the negative displacement of the threshold voltage Vth. This phenomenon can be attributed to the trapping of the photon-induced carriers into the gate dielectric/channel interface or the gate dielectric bulk. Interestingly, the negative Vth shift under photon-enhanced NBTS condition worsened in relatively humid environments. It is suggested that moisture is a significant parameter that induces the degradation of bias-stressed GIZO transistors.