Provided are: a gallium oxide single crystal composite, which can provide, for example, upon a crystal growth of a nitride semiconductor, a high-quality cubic crystal in which mixing of a hexagonal crystal is reduced to thereby realize dominant growth of a cubic crystal over hexagonal crystal, and which can be utilized as a substrate particularly suitable for epitaxial growth of cubic GaN; a process for producing the same; and a process for producing a nitride semiconductor film. The gallium oxide single crystal composite has a gallium nitride layer formed of cubic gallium nitride on a surface of the gallium oxide single crystal; the process for producing the gallium oxide single crystal composite includes subjecting the surface of gallium oxide single crystal to nitriding treatment using ECR plasma or RF plasma to form the gallium nitride layer formed of cubic gallium nitride on the surface of the gallium oxide single crystal; and further, the process for producing the nitride semiconductor film includes growing the nitride semiconductor film on the surface of the gallium oxide single crystal composite by an RF-MBE method.

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