A die stack structure including a first die, an encapsulant, a redistribution layer and a second die is provided. The encapsulant laterally encapsulates the first die. The redistribution layer is disposed below the encapsulant, and electrically connected with the first die. The second die is disposed between the redistribution layer and the first die, wherein the first and second dies are electrically connected with each other, the second die comprises a body portion having a first side surface, a second side surface and a curved side surface therebetween, and the curved side surface connects the first side surface and the second side surface.