A method of manufacturing a thin film transistor substrate may include forming a gate electrode on a base substrate, forming a gate insulation layer on the base substrate, the gate insulation layer covering the gate electrode, performing a simultaneous ultraviolet ray irradiation and thermal treatment (SUT) process by irradiating an ultraviolet ray at the gate insulation layer and supplying heat to the gate insulation layer at substantially the same time, forming an active pattern on the gate insulation layer, the active pattern overlapping the gate electrode, and forming a source electrode and a drain electrode on the gate insulation layer, the source electrode and the drain electrode being electrically connected to the active pattern.