The present invention provides a method for fabricating a single-crystalline substrate containing gallium nitride (GaN) comprising the following steps. First, form a plurality of island containing GaN on a host substrate. Next, use the plurality of islands containing GaN as a mask to etch the substrate and form an uneven host substrate. Then, perform epitaxy on the uneven host substrate to make the islands containing GaN grow in size and merge into a continuous single-crystalline film containing GaN. Finally, separate the single-crystalline film containing GaN from the uneven host substrate to obtain the single-crystalline substrate containing GaN. According to the present invention, process time can be saved and yield can be improved.