Cr doped GaN was grown by rf N-plasma molecular beam epitaxy on sapphire (0001) at a sample temperature of 700°C, Cr/Ga flux ratio was set to a value from 5% to 20%. Subsequently, scanning tunneling microscopy was performed on these surfaces. Cr incorporates on the GaN surface at 700°C at a Cr concentration of 5% and less. By increasing the Cr/Ga flux ratio to 20% in CrGaN, linear nano structures were formed on the surface, which were not observed on the bare GaN surface. The RHEED and STM studies reveal that Cr atoms form 3×3 reconstruction when 0.1 ML of Cr was deposited at room temperature on 1×l adlayer of Ga on GaN (000-1). Cr substitutes Ga on the surface when deposited at 700°C on the MBE grown GaN (000-1) surface for all the experiments, which we have performed, provided the Cr concentration is low (∼5%).