A receptacle connector for mating with the plug connector, includes an insulative housing and two rows of terminals. The housing includes a main body and a mating tongue forwardly extending from the main body and forming two opposite mating surfaces. Each of the terminals includes a front mating section and a rear mounting section. The mating tongue forms a plurality of passageways to receive the mating sections of the corresponding terminals therein, respectively. The housing further forms a plurality of holes communicatively behind the some passageways in the vertical direction for increase the impedance of the corresponding terminal on the mating sections, respectively.