Package on package structure

A package on package structure includes a first package, a plurality of conductive bumps, a second package and an underfill. The conductive bumps are disposed on a second surface of the first package and electrically connected to the first package. The second package is disposed on the second surface of the first package through the conductive bumps, and includes a semiconductor device and an encapsulating material encapsulating the semiconductor device. A shortest distance from an upper surface of the encapsulating material to an upper surface of the semiconductor device is greater than or substantially equal to twice a thickness of the semiconductor device. The underfill is filled between the first package and the second package.