A USB receptacle connector (1) comprises an insulative housing (10), a plurality of terminals (14) received in the insulative housing and a shell (11) covering the insulative housing. The frond end of the shell forms a mating port (111) for insertion of a mating plug connector (1′). Either of two sidewalls (112) of the mating port defines a contact tab (115) which makes the shell of the USB receptacle connector always contact with a shell (11′) of the mating plug connector. Thus, the two shells are continuously grounded so as to ensure reliable grounding and EMI shielding.