A thin film transistor substrate includes a base substrate, a gate electrode, a gate insulating layer, a surface treating layer, an active layer, a source electrode and a drain electrode. The gate electrode is formed on the base substrate. The gate insulating layer is formed on the base substrate to cover the gate electrode. The surface treating layer is formed on the gate insulating layer by treating the gate insulating layer with a nitrogen-containing gas to prevent leakage current. The active layer is formed on the surface treating layer to cover the gate electrode. The source electrode and the gate electrode that are spaced apart from each other by a predetermined distance are formed on the active layer.