Rapid thermal annealing (RTA) of the cubic GaN (c-GaN) film grown on GaAs (0 0 1) substrate by molecular beam epitaxy shows that the surface morphology is improved. The average root-mean-square (RMS) surface roughness measured by atomic force microscopy (AFM) decreased from 4.86 nm on the as-grown film to 3.57 nm after a 1000 degreesC annealing. There is no shift but Raman features show an increase in intensity and changes in the full-width at half-maximum (FWHM) observed before and after the annealing. High-resolution X-ray diffraction (HRXRD) is used in two-dimensional (2D) triple axis mapping mode to characterize the c-GaN epitaxial films before and after the annealing process.