A stack package may include a first chip, a second chip, a through silicon via (TSV) and an interface circuit unit. The first chip may include a first internal circuit unit driven by an internal voltage. The second chip may be stacked over the first chip. The second chip may include a second internal circuit unit driven by the internal voltage. The TSV may be electrically coupled between the first chip and the second chip. The interface circuit unit may be arranged in the first chip and the second chip. The interface circuit unit may be coupled to the TSV. A portion of the interface circuit unit may be received a variable voltage different from the internal voltage as a driving voltage.