An illumination system includes a light source assembly including a first excitation light source, a second excitation light source, a dichroic element, a wavelength conversion element, a first collimating element and a second collimating element. The first and second excitation light sources provide first and second excitation beams, respectively. The dichroic element is disposed between the first excitation light source and the second excitation light source. The second excitation beam is transmitted toward the first excitation light source through the dichroic element. The wavelength conversion element is disposed between the first excitation light source and the dichroic element. The wavelength conversion element converts the first and second excitation beams into a wavelength conversion beam and transmits it toward the dichroic element.