Gallium Nitride (GaN) bulk crystal growth is a challenging process due to its high melting temperature, low sublimation/decomposition temperature, very high equilibrium nitrogen vapor pressure, and low solubility. A GaN bulk crystal can be grown from the vapor phase by either evaporation of the gallium (Ga) metal or sublimation of GaN powder in an ammonia (NH3) atmosphere in a temperature-controlled growth furnace. The Sublimation Sandwich Technique (SST) is one of the techniques which utilizes reaction of gallium vapors with ammonia. Not only a high growth rate (up to 1mm/h) but also the intensity and uniformity of the deposition can be achieved and controlled through changing temperatures on the source, and the substrate, and the distance between them. A GaN growth model using SST has been developed to investigate the effects of the incoming velocity, source temperature, temperature difference, and the gap width on the growth cell during the growth process.