Structural and optical studies were made on the (0 0 1)GaAs surfaces nitrided at 700 degreesC for 15 min in the pressure between 0.01 and 0.5 Torr, plasma-assisted NH3 gas atmosphere. Reflection high-energy electron diffraction (RHEED) and PLS results indicated that the surface nitrided in 0.01 Torr NH gas consisted of cubic gallium nitride (c-GaN) with epitaxial orientation: (0 0 1), [1 1 0]c-GaN//(0 0 1), [1 1 0]GaAs, showing PL spectrum with sharp, blue emission of lambda = 440 nm and a broad peak approximately 470 nm at room temperature. The surface nitrided in 0.05 Torr NH gas was composed of a mixture of cubic and hexagonal GaN, showing blue photoluminescence (PL) emission with wavelength, approximately 440 nm at room temperature. The surface nitrided in 0.5 Torr NH gas consisted of polycrystalline hexagonal GaN, showing blue PL emission with wavelength, approximately 440 nm at room temperature. Results of depth profile of the nitrided surface, by AES with sputtering technique using argon gas, indicated that the nitrided layers varies approximately 50-1000 Angstrom in thickness, depending on the NH3 gas pressure, as a GaNAs alloy layer. These results are in reasonable agreement with theoretical prediction on the band-gap energies in GaN-GaAs alloy system.