An illumination system includes a blue light source, an excitation light source, a first light splitting element and a wavelength conversion element. The blue light source provides a blue light beam. The excitation light source provides an excitation light beam. The first light splitting element and the wavelength conversion element are disposed on transmission paths of the blue light beam and the excitation light beam. The wavelength conversion element includes a diffusion region and a wavelength conversion region. In a first timing period, the excitation light beam passing through the first light splitting element is transmitted to the wavelength conversion region to be converted into an excited light beam. In a second timing period, the blue light beam passing through the diffusion region is transmitted to the first light splitting element. The excited light beam includes a first red light beam and a green light beam.