An illumination system having a light source, an integration rod and a color wheel is provided. The light source is suitable for providing a light beam. The light integration rod and the color wheel are disposed in the transmission path of the light beam. The color wheel is disposed between the light source and the light integration rod. The light integration rod has a light input end adjacent to the color wheel and a light output end opposite the light input end. The light integration rod has two indentations at the light input end to form an airflow channel. The noise caused by the rotating color wheel is reduced through the airflow channel.