GaN: Mg films have been grown on sapphire at low temperature by metal-organic chemical vapor deposition, the properties of different source flux GaN: Mg materials were studied. When the molar ratio of CP2Mg and TMGa is between 1.4×10-3 and 2.5×10-3, the quality of crystal was improved with the increasing molar ration, and the hole concentration was increased linearly. When the molar ratio is 2.5×10-3，the concentration is equal to that of the film grown at higher temperature, and the surface morphology is more coarser. Taking the p-GaN layer with molar ratio of CP2Mg and TMGa of 2.5×10-3　as the light-emitting diode, when the inject current is 20 mA, the output light power was increased by 17.2%.