We demonstrate the growth of GaN by molecular beam epitaxy with warm ammonia as a nitrogen source. Ammonia gas is heated by the tungsten filament located at the open end of the gas-tube installed in the growth chamber. By using this simple structure, the multiple collisions of molecules within the heater, thus the generation of nitrogen molecule, can be suppressed. The crystalline quality of the grown GaN layer is significantly improved by introducing the warm ammonia. This effect can be explained by the enhancement of the two-dimensional growth due to the active nitrogen species such as radical NH2* generated by cracking ammonia molecule. (C) 2009 Elsevier B.V. All rights reserved.