A package stack device includes a first package structure having a plurality of first metal posts and a first electronic element disposed on a surface thereof, a second package structure having a plurality of second metal posts and a second electronic element disposed on opposite surfaces thereof, and an encapsulant formed between the first and second package structures for encapsulating the first electronic element. By connecting the first and second metal posts, the second package structure is stacked on the first package structure with the support of the metal posts and the encapsulant filling the gap therebetween so as to prevent warpage of the substrate.