An illumination system includes a light source module, a chromaticity-adjusting light source and a light-combining element. The light source module provides a first color light beam, a second color light beam and a third color light beam. The chromaticity-adjusting light source provides a chromaticity-adjusting light beam. The light-combining element is disposed on the transmission paths of the chromaticity-adjusting light beam and the color light beams provided by the light source module to combine the color light beams with the chromaticity-adjusting light beam into an illumination beam.