An illumination system includes a first light source providing a first color beam, a first beam-splitting element which disposes on a transmission path of the first color beam and separates the first color beam into a first sub-beam and a second sub-beam, an optical wavelength conversion element which receives the first sub-beam and converts the first sub-beam into a second color beam and reflects the second color beam back to the first beam-splitting element, a second light source providing a third color beam, and a second beam-splitting element which is located between the second light source and the first beam-splitting element to reflect the second sub-beam. The third color beam passes through the second beam-splitting element and is transmitted to the first beam-splitting element. The second sub-beam, the second color beam and the third color beam are combined into an illumination beam.