In this work, we report that the electrical characteristics of Indium-Gallium-Zinc-Oxide Thin Film Transistors (IGZO-TFTs) can be improved by Mg doping in the IGZO layer. Several composite IGZO/Mg-doped IGZO channel structures were employed. With appropriate Mg-doped IGZO thickness, the electrical properties such as mobility, subthreshold swing (S.S.) and on/off ratio can be significantly improved by Mg doping. Mg doping in IGZO layer can stabilize device performance whatever the device is stressed under positive gate bias or negative gate bias with illumination. By X-ray photoelectron (XPS) analysis, we observe the presence of Mg can change the oxygen bonding states and influence the properties of IGZO layer. The atomic percentage of Mg in the IGZO layer is about 6.53%. As a result, Mg doping is a useful method to enhance the performance and stability of IGZO-TFTs.