A thin film transistor substrate includes a gate electrode disposed on a base substrate, an active pattern overlapping the gate electrode, a source metal pattern including both a source electrode disposed on the active pattern and a drain electrode spaced apart from the source electrode, a buffer layer disposed on the source metal pattern and contacting the active pattern, a first passivation layer disposed on the buffer layer and a second passivation layer disposed on the first passivation layer. The density of hydrogen in the buffer layer is greater than the density of hydrogen in the first passivation layer and less than the density of hydrogen in the second passivation layer.