Abstract: We experimentally addressed the influence of the thickness-ratio of SiO_x/TaO_xin the gate insulators (GI) on the properties of the flexible amorphous InGaZnO thin film transistors (a-IGZO TFTs), fabricated at room temperature by magnetron sputtering on polyimide (PI) substrate.The preliminary results show that when it comes to the performance and bias-stress stability of TFTs, the SiO_x/TaO_xlayers outperformed the conventional SiO_xlayer.To be specific, the TaO_xlayer grew much faster than the SiO_xlayer; as the TaO_xthickness increased, the GI dielectric constant increased, accompanied by the decrease of GI surface roughness.For instance, the 300 nm SiO_xand 300 nm TaO_xlayers increased the relative GI dielectric constant up to 10, resulting in a larger on-current, a lower threshold voltage, a little smaller off-current but a bit worse bias-stress stability of the corresponding a-IGZO TFTs.