A method of fabricating a thin film transistor substrate includes forming a gate wiring on an insulating substrate and forming a gate insulating layer on the gate wiring; performing a first hydrogen plasma treatment with respect to the gate insulating layer; forming a first active layer with a first thickness at a first deposition rate on the gate insulating layer; performing a second hydrogen plasma treatment with respect to the first active layer; and forming a second active layer with a second thickness greater than the first thickness at a second deposition rate greater than the first deposition rate, on the first active layer.