A board-to-board connector assembly is used to electrically connect a first circuit board and a second circuit board and has a receptacle connector and an engaging element assembled together. The receptacle connector includes an insulative housing and a plurality of conductive terminals received in the insulative housing. An engaging groove is defined in the receptacle connector and has the same shape as the engaging element for receiving the engaging element therein. The engaging element forms positioning portions thereon for guiding the receptacle connector to mate with a circuit board accurately and facilitating the terminals to contact electrodes on the circuit board firmly. The engaging element is firmly and accurately retained in the receptacle connector to assure reliable signal transmission between the first circuit board and the second circuit board. Simultaneously the overall height of the board-to-board connector assembly is effectively decreased.