Disclosed is a method of fabricating nitride semiconductors in a MOCVD reactor. GaN is first deposited on an inner wall of the MOCVD reactor, and a sapphire substrate is loaded into the MOCVD reactor. The sapphire substrate is heated and etching gas is injected into the MOCVD reactor. NH3 gas is injected into the MOCVD reactor to nitrify the surface of the sapphire substrate. A nitride semiconductor layer is grown on the nitrified sapphire substrate. By surface-reforming the sapphire substrate and then growing the nitride semiconductor layer on the surface-reformed sapphire substrate via MOCVD without formation of a low temperature buffer layer, an excellent nitride semiconductor structure can be realized. In this circumstance, the nitride semiconductor layer for example of GaN can be grown effectively on the surface-treated sapphire substrate because GaN deposition occurs on the sapphire substrate while it is etched.