A heat dissipation device includes a heat sink with a plurality of fins, a fan mounted on a side of the heat sink with a space formed between the heat sink and the fan, and at least one airflow guiding board formed by one end of at least one of the fins. The airflow guiding board extends through the space to connect with the fan. The airflow guiding board can guide the airflow generated by the fan to the heat sink, so as to reduce the loss of the airflow from the edge of the heat sink, and enhance the effect of heat dissipation. Alternatively, the airflow guide board can also function to guide the airflow to cool other heat sinks.