A portable electronic device includes a casing, an electronic component disposed in the casing, and a thermal module for dissipating heat of the electronic component. The thermal module includes a heat pipe, a fin unit, and a transverse fan. The heat pipe includes an evaporating section thermally attached to the electronic component, and a condensing section thermally attached to the fin unit. The transverse fan is arranged in the casing for generating an airflow through the electronic component and the fin unit. The transverse fan includes a rotor, and a sidewall on a floor of the casing and surrounding the rotor. The sidewall defines an air inlet and an air outlet therein. The air outlet faces to the fin unit. The air inlet faces to the electronic component.