The structural and electrical changes of the thin GaN layer prepared on a (001) GaAs substrate before and after rapid thermal treatment have been investigated by Raman scattering, glancing X-ray diffraction, and current-/capacitance-voltage measurements. Thin GaN layers grown on a (001) GaAs substrate by metal organic chemical vapor deposition were shown to be composed mostly of wurtzite-structured GaN (alpha-phase), whereas zincblende GaN (beta-phase) was present near the interface between the thin GaN layer and the GaAs substrate. Rapid thermal treatment at 750-850 degreesC caused a structural phase of the GaN to evolve; the higher fraction of beta-phase was still maintained at the hetero-interfacial region due to the strong influence of the (001) GaAs matrix. By contrast, several mixed structures of beta-(111) and alpha-(10 (1) over bar1) appeared near the surface of the GaN layer that was farthest from the interface region. After the thermal treatment, the electrical properties of the GaN/GaAs hetero-interface were those of a quasi-metal-insulator-semiconductor. The detailed structural changes and the related electrical characteristics of the thin GaN layer on (001) GaAs are discussed.