A substrate 1 for growing an electro-optical single crystal thin film in which two or more layers of buffer layers 3, 4, and 5 for buffering lattice mismatch between Si and BTO are formed on an Si (001) substrate 2 is provided as a substrate for growing an electro-optical single crystal thin film which can obtain an electro-optical single crystal thin film of BTO single crystal thin film 6 etc. with a large size and a very high quality.