A portable electronic device, which including a casing, a circuit board, a fan and a heat dissipating device, is disclosed. The circuit board is disposed in the casing and includes at least one electronic component thereon. The fan is disposed in the casing. The heat dissipating device is disposed in the casing and near the side of an air outlet of the fan. Gaps formed between the outer surfaces of the heat dissipating device and the inner surfaces of the casing as air flow channels. The portable electronic device isolates heat conducted to the casing of the portable electronic device via the gaps.