A heat sink includes a first array of fins defining a plurality of first channels therebetween, a second array of fins disposed at opposite sides of the first array of fins, and a plurality of grooves extending through the first array of fins and the second array of fins. The second array of fins defines a plurality of second channels therebetween. The grooves are intersected with the first channels and the second channels. A bottom extremity of each groove has a curved shape facing upwardly and laterally. A portion of an airflow generated by a fan mounted on a top of the heat sink flows laterally and downwardly through the grooves to leave the heat sink.