Silverman proved the following theorem: Let E be an elliptic curve over 〖WTHZ〗C〖WTBZ〗 with complex multiplication 〖WTHT1〗OK, where 〖WTHT1〗OK is the ring of integers of an imaginary quadratic field K, then the Hilbert class field of K is generated by the j invariant of E over K. In this paper we give a simple proof of this fundamental theorem.