A substrate for manufacturing a GaN-based semiconductor device is provided to enhance electrical characteristics by preventing warpage or breakdown of a substrate due to a thermal expansion coefficient difference. A substrate(100) for manufacturing a GaN-based semiconductor device is made of Al2O3. The wafer is used as an epi-wafer. Two or more grooves(120a) are formed on a surface(120) which is opposite to a growing surface(110) of the GaN-based semiconductor device. The desired thickness of the substrate is 350Å and more. The desired diameter of the substrate is 4 inch and more. The grooves are formed to perform a function for dispersing stress such as tensile force or compressive force. The stress such as tensile force or compressive force is dispersed through the grooves in order to prevent the warpage or the breakdown of the substrate.
