A lamp includes a substrate having first and second sides. At least one lighting element is mounted on the first side. An air-guiding member is mounted to the substrate. An airflow chamber is formed between the air-guiding member and the second side of the substrate. The air-guiding member includes a plurality of air-guiding holes in communication with the air-guiding chamber. A plurality of heat-dissipating fans is respectively mounted to the air-guiding holes. A control unit is electrically connected to the heat-dissipating fans. The control unit controls operation timing and operation modes of the heat-dissipating fans so that the lamp has a plurality of heat-dissipating modes.