A board-to-board connector includes a socket and a plug. The socket has a dielectric housing and a plurality of receiving terminals received in the dielectric housing. The dielectric housing defines a recess and a plurality of sidewalls surrounding the recess. At least one of the sidewalls defines at least one fixing groove connecting to the recess and passing through a top portion of the sidewall. A central portion of two opposing sides of the fixing groove protrudes towards each other to form two facing fixing projections to make the central portion of the fixing groove narrower than two ends thereof. The plug engages into the recess of the socket. The plug has a dielectric body and a plurality of inserting terminals received in the dielectric body. The dielectric body protrudes sideward to form at least one locating lump corresponding to the fixing groove. The locating lump passes from between the two fixing projections by extrusion therebetween and then is blocked by the two fixing projections.