An illumination system and a projection device comprising the same are provided. The projection device further comprises an imaging system. The illumination system comprises a first light source, a dichroic element and a wheel plate. The dichroic element comprises a first dichroic portion and a second dichroic portion. The first light source provides a first wave band light, which is reflected to the wheel plate by the first dichroic portion. When being projected onto a wave band transforming area of the wheel plate, the first wave band light will be transformed into a second wave band light by the wave band transforming area and then emits through the first dichroic portion and the second dichroic portion. When being projected onto a reflecting area of the wheel plate, the first wave band light will be reflected by the reflecting area to emit through the second dichroic portion.