A three dimensional integrated circuit (3DIC) includes a first substrate and a heat spreading structure embedded in the first substrate. The 3DIC further includes a die electrically connected to the first substrate, wherein the die is thermally connected to the heat spreading structure. The 3DIC further includes a plurality of memory units on the die, wherein the die is between the plurality of memory units and the first substrate, and the plurality of memory units is thermally connected to the heat spreading structure by the die. The 3DIC further includes an external cooling unit on the plurality of memory units, wherein the plurality of memory units is between the die and the external cooling unit, and the die is thermally connected to the external cooling unit by the plurality of memory units.