Low-temperature growth of GaN is very attracting for the application to light emitting devices grown on Si substrates because it prevents the melt-back reaction between Ga and Si substrates. The low-temperature growth of GaN by compound-source molecular beam epitaxy (CS-MBE) has been reported. In the previous report, GaN powders were used as a source and no additional nitrogen source was introduced during the growth. At present, its growth mechanism is unclear. In this paper, CS-MBE of GaN layers using GaN and ammonia as sources is discussed. Especially, the reduction of excess Ga in GaN layers by introducing ammonia supply is discussed based on their refraction high-energy electron diffraction (RHEED) patterns and x-ray photoelectron spectroscopy (XPS) spectra.