A thin-film transistor (TFT) substrate has improved electrical properties and reduced appearance defects and a method of fabricating the TFT substrate, are provided. The TFT substrate includes: gate wiring which is formed on a surface of an insulating substrate; oxide active layer patterns which are formed on the gate wiring and include an oxide of a first material; buffer layer patterns which are disposed on the oxide active layer patterns to directly contact the oxide active layer patterns and include a second material; and data wiring which is formed on the buffer layer patterns to insulatedly cross the gate wiring, wherein a Gibbs free energy of the oxide of the first material is lower than a Gibbs free energy of an oxide of the second material.