A diamond substrate and a method for fabricating the same are provided wherein a SiC layer is formed on a lower surface of a diamond layer for preventing the diamond layer from being deformed after the process of forming the diamond substrate, and then a semiconductor layer is formed on the diamond layer or directly formed on the surface of the SiC layer. Thereby, the lattice mismatch between the diamond film layer and the semiconductor layer is mitigated by the SiC layer, and the crystalline quality of the semiconductor layer is improved, the fabricating process of the diamond substrate is simplified, and the performance and stability are enhanced.