An electrical connector has an insulative elongated housing retaining a plurality of contacts therein. The housing includes an upper wall, a lower wall and a rear wall to commonly form a receiving slot. The upper wall forms a plurality of upper passageways and the lower wall forms a plurality of lower passageways to receiving the corresponding upper contacts and lower contacts therein. Each contact includes a mating section extending into the mating space and a mounting leg extending outside of the housing. The housing forms a plurality of cavities in the upper wall, the lower wall and the rear wall to relative evenly adjust the thickness of the different positions in the cross-section of the housing so as to evenly molding the housing without improper deformation/warpage when the housing is solidified after molding.