The stack package includes a first semiconductor package and a second semiconductor package. The first semiconductor package includes a first substrate having a first modulus and at least one semiconductor chip mounted on the first substrate. The second semiconductor package stacked on the first semiconductor package and includes a second substrate having a second modulus and at least one semiconductor chip mounted on the second substrate. The second modulus is less than the first modulus. Even in the event that the first semiconductor package is under severe warpage due to a temperature change, the flexible second substrate, which includes e.g., polyimide or poly ethylene terephthalate, of the second semiconductor package may be less sensitive to the temperature change, thereby improving reliability of the stack package.