An apparatus for fabricating a GaN single crystal and a fabrication method for producing GaN single crystal ingot are provided. The apparatus includes: a reactor including a ceiling, a floor and a wall with a predetermined height encompassing an internal space between the ceiling and the floor, wherein the ceiling is opposite to the floor; a quartz vessel on the floor containing Ga metal; a mount installed on the ceiling on which a GaN substrate is mounted, the GaN substrate being opposite to the quartz vessel; a first gas supplying unit supplying the quartz vessel with hydrogen chloride (HCl) gas; a second gas supplying unit supplying the internal space of the reactor with ammonia (NH3) gas; and a heating unit installed in conjunction with the wall of the reactor for heating the internal space, wherein the lower portion of the internal space is heated to a higher temperature than the upper portion.