A method for fabricating a thin film transistor comprises disposing a semiconductor layer including a channel area, and a first and second electrode areas, disposing a gate insulating layer covering the semiconductor layer, disposing a diffusion barrier layer covering the gate insulating layer, disposing first and second electrode holes corresponding to the first and second electrode areas, and disposing a gate electrode overlapping the channel area, first and second electrodes electrically connected to the first and second electrode areas through the first and second electrode holes. The disposing of the first electrode hole, and the second electrode hole includes patterning the diffusion layer with a patterning mask disposed on the diffusion barrier layer. The disposing of the gate electrode and the first and second electrodes may include disposing a conductive material layer covering the diffusion barrier layer, and patterning the conductive material layer.