An illumination system including an excitation light source, a transparent substrate, a dichroic film, a first color wheel element and a light uniforming element are provided. The excitation light source emits a first color light. The first color wheel element includes a wavelength conversion material. The first color wheel element has a first side and a second side. The first color light passes through a solid structure of the transparent substrate and the dichroic film, and is transmitted to the first color wheel element and converted into a second color light by the wavelength conversion material. A first part of the second color light is transmitted towards the dichroic film, and is reflected back to the first color wheel element by the dichroic film to enter the light uniforming element. A second part of the second color light passes through the first color wheel element to enter the light uniforming element.