An illumination system including an excitation light source, a first light splitting element, a wavelength conversion element, a second light splitting element, a third light splitting element and a fourth light splitting element is provided. The excitation light source provides a light beam. The first light splitting element allows the light beam to pass through to generate a first light beam or reflects it to generate a second light beam. The wavelength conversion element is located on a light path of the first light beam. The second light splitting element allows the first light beam to pass through to be transmitted to the wavelength conversion element to generate a third light beam and reflects the third light beam. The third light splitting element reflects the second light beam. The fourth light splitting element allows the second light beam to pass through and reflects the third light beam.