An electrical connector includes: an insulative housing having a row of slots defined by plural dividing walls thereof, and plural contacts secured in the insulative housing and each including a securing portion, a front contacting portion disposed in a corresponding slot, and a rear tail, wherein the plurality of dividing walls includes a group of first dividing walls each having a first length and a group of second dividing walls each having a second length along a front-to-back direction, and the first length is shorter than the second length to expose to air a larger extent of the front contacting portions of associated contacts disposed in the slots defined by the group of first dividing walls with respect to the front contacting portions of associated contacts disposed in the slots defined by the group of second dividing walls.