Efficiency and lifetime of GaN based light emitting diodes and laser diodes can be improved by proper choice of substrate or deliberate modification of the substrate surface before deposition. Buffer growth or nitridation is the usual choices of surface modifications for GaN deposition to improve crystal duality and optical properties. It was our intention to study a possibility that a reactive ion beam (RIB) pretreatment of the sapphire substrate at room temperature could substitute the nitridation process at high temperature above 1000 degrees C. The optical property of GaN films deposited on the sapphire surfaces pretreated by the reactive ion beam has improved significantly. Current observations clearly demonstrates that the RIB pretreatment of the sapphire surface can be used to improve the GaN films grown by metalorganic chemical vapor deposition (MOCVD).