An electrical connector assembly includes a metallic cage defining a receiving space therein, a receptacle connector unit received in the receiving space and including a pair of mating ports, and a number of terminal wafers stacked with one another with corresponding contacting sections exposed upon the mating ports and tail sections to be mounted on a printed circuit board on which the receptacle connector unit is mounted, and a middle heat transfer unit located between the two mating ports in a vertical direction and including a heat base linked with a pair of heat pipes on two sides, the pair of heat pipes extending through a rear of the cage and connected to a rear heat sink unit located behind the rear of the cage. Each of the pair of heat pipes is directly contacted with at least two different surfaces of the heat base.