Abstract: A thin film transistor includes a gate electrode, a gate insulating layer on the gate electrode, a semiconductor on the gate insulating layer, and a drain electrode and a source electrode on the semiconductor and spaced apart from each other. Each of the drain electrode and the source electrode includes a first metal diffusion preventing layer which prevents diffusion of metal atoms, and a second metal diffusion preventing layer on the first metal diffusion preventing layer. At least one of the first and second metal diffusion preventing layers includes grains in a columnar structure, which are in a direction substantially perpendicular to a lower layer. First grain boundaries of the first metal diffusion preventing layer and second grain boundaries of the second metal diffusion preventing layer are substantially discontinuous in a direction perpendicular to the semiconductor.