An electrical connector assembly includes a first and a second connector (1, 2) each including an insulating housing and a plurality of terminals retained in the housing. The housing has a front mating portion, a rear face and a pair of first retaining portions at opposite ends of the rear face of the insulating housing. The terminals include leg portions extending out of the rear face of the housing. A receiving cavity (15) is defined between the pair of retaining portions of the first connector. A transition board (3) is vertically received in the receiving cavity of the first connector and the two pairs of retaining portions are engaged with each other to assembly the two connectors together.