A heat dissipation device includes a heat sink and an LED module attached to the heat sink. The heat sink includes a base and a plurality of fins mounted on the base. A plurality of channels is defined between the fins of the heat sink and slits are defined in two opposite side edges of the base. The slits extend through the base and corresponding fins and cross with corresponding channels. A plurality of grooves is defined in the fins opposite to the LED module. Each of the grooves interconnects corresponding two aligned slits.