The electrical stability of amorphous InGaZnO (a-IGZO) TFTs with three different channel layers was investigated. Compared with the single channel layer, the a-IGZO TFT with double stacked channel layer showed the lowest threshold voltage shift with slightly change in field effect mobility and sub-threshold swing under positive and negative gate bias stress tests. Moreover, sputtered SiNx thin film was served as passivation layer where the Vth shift in bias stress effect evidently became less. It was found that the passivated a-IGZO TFT with double stacked channel layer still exhibited the best stability. The results prove that the stability of a-IGZO TFTs can be effectively improved by using double stacked channel layer and passivation layer.