Disclosed is a ball grid array substrate having a window formed on a core material instead of a thin core material, and wherein a semiconductor chip is mounted thereon, thereby reducing the thickness of a package, and a method of fabricating the same. The ball grid array substrate comprises a first external layer which includes first circuit patterns, wire bonding pad patterns, and a window corresponding in size to a first chip mounted therein and wherein the chip is wire-bonded to the wire bonding pad patterns. A second external layer includes second circuit patterns, a portion corresponding in position to the window of the first external layer, and solder ball pad patterns. Second chips mounted on the solder ball pad patterns. An insulating layer interposed between the first and second external layers. The window is formed though the insulating layer at a position corresponding to the window of the first external layer.