A heat dissipation device includes a heat sink having a plurality of fins defining a plurality of passageways therebetween. The passageways have an inlet and an outlet at front and rear sides of the heat sink. Two adjusting members are mounted at two lateral sides of the heat sink. A fan is located at the inlet of the passageways of the heat sink for providing airflow to the heat sink. A guiding member is positioned at the outlet of the passageways of the heat sink for guiding the airflow to electronic components around the heat dissipation device. The guiding member engages the adjusting members and can move vertically relative to adjusting members, thereby adjusting a height of the guiding member along a height of the heat sink.