A thin film transistor substrate includes a gate electrode on a base substrate, an active pattern on the gate electrode, a source electrode on a first end of the active pattern, a drain electrode on a second end of the active pattern, an organic insulation layer on the source electrode and the drain electrode, and a transparent electrode contacting the drain electrode through a contact opening in the organic insulation layer. The drain electrode is spaced from the source electrode. The organic insulation layer includes a first thickness portion around the contact opening and a second thickness portion adjacent to the first thickness portion. The second thickness portion has a thickness greater than that of the first thickness portion.