Provided is a method for preparing a substrate for growing gallium nitride and a gallium nitride substrate. The method includes performing thermal cleaning on a surface of a silicon substrate, forming a silicon nitride (Si3N4) micro-mask on the surface of the silicon substrate in an in situ manner, and growing a gallium nitride layer through epitaxial lateral overgrowth (ELO) using an opening in the micro-mask. According to the method, by improving the typical ELO, it is possible to simplify the method for preparing the substrate for growing gallium nitride and the gallium nitride substrate and reduce process cost.