In this study, we introduced a new growth method of gallium nitride (GaN), and demonstrated maskless selective epitaxy of cubic GaN on GaAs (100) substrates using a Ga low energy focused ion beam (Ga-LEFIB) and dimethylhydrazine (DMHy) [(CH3)(2)N2H2] as Ga and N sources, respectively. A Ga-LEFIB with an incident energy of 30-300 eV and DMHy were supplied simultaneously, and GaN films were grown selectively only where the focused Ga ion beam was irradiated. We have found that GaN growth was strongly dependent on growth temperature and DMHy pressure, while surface nitridation time of GaAs (100) had little effect on crystalline quality. The optimized growth conditions of GaN obtained:in this experiment were similar to 610 degrees C growth temperature with a DMHy pressure of 6.0 X 10(-6) Torr at an ion beam incident energy of 30 eV.