A heat sink includes a thermally conductive base, a plurality of thermally conductive fins extending from the base, and a guiding member. The base has a top surface from which the fins extend, and a bottom surface for being attached onto a heat-generating electronic component. A channel is formed between every two fins. Each of the fins is generally rectangular with the top edge being shorter than the bottom edge. Each of the fins comprises an upright side edge perpendicular to the top surface of the base, and an opposite streamlined side edge. The upright side edges and the streamlined side edges cooperate with the channels to form an air inlet for airflow flowing into the channels and an air outlet for airflow flowing out of the channels, respectively. The guiding member is attached at the streamlined side edges of the fins for guiding airflow flowing out of the channels.