An illumination device includes a first light-emitting module, a second light-emitting module and an airflow-guiding structure. The second light-emitting module is adjacent to the first light-emitting module. The airflow-guiding structure includes a receiving casing, a first air inlet pipe in air communication with the receiving casing, and a first air outlet pipe in air communication with the receiving casing. The first light-emitting module is received inside the airflow-guiding structure. When external air flows into the receiving casing through the first air inlet pipe by natural convection, the external air inside the receiving casing is sterilized by a sterilization light source provided by the first light-emitting module, and the external air that has been sterilized by the sterilization light source flows out of the receiving casing through the first air outlet pipe by natural convection, and is then discharged out of the illumination device.