A receptacle connector includes an insulative receptacle housing and a plurality of receptacle contacts retained in the receptacle housing. The plug connector includes an insulative plug housing and a plurality of plug connectors retained in the plug connector. The receptacle housing includes a rear wall extending along a lengthwise direction, and a pair of side walls extending from two opposite ends of the rear wall along the front-to-back direction perpendicular of the lengthwise direction, to commonly form a receiving space thereamong. The receiving space forms a front opening and an upper opening for communicating with an exterior. The receptacle contacts are retained in the rear wall and extend into the receiving space. The plug connector is adapted to be rearwardly inserted into the receiving space along the front-to-back direction via the front opening, or be downwardly and rotationally inserted into the receiving space via the upper opening.