A package stack device includes a first package structure having a plurality of first metal posts and a first electronic element, a second package structure having a plurality of second metal posts and a second electronic element, and an encapsulant formed between the first and second package structures to encapsulate the first electronic element. By connecting the second metal posts to the first metal posts, respectively, the second package structure is stacked on the first package structure with the support of the metal posts. Further, the gap between the two package structures is filled with the encapsulant to avoid warpage of the substrates.