A method of fabricating an array substrate includes forming a first metal layer, a gate insulating material layer and an oxide semiconductor material layer on a substrate; heat-treating the substrate having the oxide semiconductor material layer at a temperature of about 300 degrees Celsius to about 500 degrees Celsius; patterning the oxide semiconductor material layer, the gate insulating material layer and the first metal layer, thereby forming a gate electrode, a gate insulating layer and an oxide semiconductor layer; forming a gate line connected to the gate electrode and made of low resistance metal material; forming source and drain electrodes, a data line and a pixel electrode, the source and drain electrodes and the data line having a double-layered structure of a transparent conductive material layer and a low resistance metal material layer, the pixel electrode made of the transparent conductive material layer.