A method of growing a III group nitride single crystal by using a metal-organic chemical vapor deposition (MOCVD) process, the method including: preparing an r-plane (1-102) substrate; forming a nitride-based nucleation layer on the substrate; and growing a nonpolar a-plane nitride gallium single crystal on the nitride-based nucleation layer while altering increase and decrease of a ratio of V/III group to alternate a horizontal growth mode and a vertical growth mode.