A method of manufacturing a free-standing gallium nitride (GaN) substrate, by which a free-standing GaN substrate can be manufactured without warping or cracks. The method includes the steps of collecting polycrystalline GaN powder that is deposited in a reactor or on a susceptor in a process of growing single crystalline GaN, loading the collected polycrystalline GaN powder into a forming mold, preparing a polycrystalline GaN substrate by sintering the loaded polycrystalline GaN powder, and forming a single crystalline GaN layer by growing single crystalline GaN over the polycrystalline GaN substrate. It is possible to reduce warping and cracks that are caused, due to the difference in the coefficient of thermal expansion, during the growth or cooling of single crystalline GaN in the process of manufacturing the free-standing GaN substrate.