The kinetics of GaN growth by MBE using ammonia as the reactive nitrogen source is studied both under Ga-rich and N-rich conditions. It is shown that adsorption site blocking by Ga and N atoms as well as by surface NH, complexes is a crucial factor to control thr growth rate of the crystal. Use of N-rich growth conditions easily achieved in ammonia MBE allows one to increase the GaN growth temperature at least by approximate to 80 to 90 K compared to plasma-enhanced MBE, that is favorable to improve optical properties of the grown material.