A package structure is provided, which includes a dielectric layer having opposing first and second surfaces, and through holes penetrating the surfaces; a strengthening layer formed on the first surface; a circuit layer formed on the second surface, and having wire bonding pads formed thereon and exposed from the through holes, and ball pads electrically connected to the wire bonding pads; a first solder mask layer formed on the first surface and the strengthening layer, and having first apertures formed therethrough for exposing the wire bonding pads; a second solder mask layer formed on the second surface and the circuit layer, and having second apertures formed therethrough for exposing the ball pads; and a semiconductor chip disposed on the first solder mask layer and electrically connected via conductive wires to the wire bonding pads exposed from the through holes. The strengthening layer ensures the steadiness of the chip to be mounted thereon without position shifting.