An illumination system including a first integrator, a first dichroic element, a second dichroic element, a first light source, and a second light source is provided. The first integrator has a first light incident end and a first light exit end. The first and the second dichroic elements are respectively disposed at two different parts adjacent to the first light incident end. The first dichroic element, the second dichroic element, and the first light incident end are arranged along a triangular track. The first light source emits a first color light. The second light source emits a second color light. The first color light passes through the first dichroic element and is partially reflected by the second dichroic element to the first light incident end. The second color light passes through the second dichroic element and is partially reflected by the first dichroic element to the first light incident end.