A heat dissipation device (40) is used for dissipating heat generated by a plurality of LEDs (15) mounted on a circuit board (12). The heat dissipation device comprises two heat sinks (30) mounted on the circuit board via a heat spreader (20). Each of the two heat sinks comprises a plurality of fins (300) stacked together. A plurality of short walls (350) are formed at two opposite lateral sides of the two heat sinks. A plurality of openings (330) are defined below the short walls and opened laterally. A plurality of vertical cavities (360) are defined by the two heat sinks and communicate with the openings (330), respectively. The cavities in the two heat sinks are alternately arranged.