An electrical connector assembly includes a first connector and a second connector mating with the first connector in a mating direction. The first connector includes a mating portion defining two long walls opposite to each other and two short walls disposed oppositely, the long and short walls are connected to each other together to form a mating cavity. The second connector includes a mating tongue extending forwardly and a second shielding shell defining a mating frame surrounding the mating tongue. Wherein one of the long walls defines a pair of fool-proof portions extending outwardly from both sides thereof, the mating frame defines a pair of fool-proof slots and engaging with the fool-proof portions so as to prevent the first connector from mismating the second connector.