An electrical connector assembly comprises a first connector and a second connector, the first connector comprises a first insulative housing defining a guide groove extending from the top face to the bottom face, and a plurality of first terminals received in the first passageways; The second connector comprises a second insulative housing defining a guide post adapted to mate with the guide groove for fastening the first connector and the second connector, and a plurality of second terminals received in the second passageways, wherein the guide post penetrates through the guide groove and the transition portion of each first terminal respectively engages with the engagement section of corresponding second terminal when assembled together.