An illumination system includes a wavelength-transforming device, a solid-state light-emitting element and a filter wheel. The wavelength-transforming device comprises a segment and a wavelength-transforming element disposed on the segment. The solid-state light-emitting element emits first color light in first waveband region to the wavelength-transforming device. The filter wheel is disposed on one side of the wavelength-transforming device for filtering lights via rotating. The first color light is excited as second color light in second waveband region by the wavelength-transforming device, so that the second color light is outputted with the rest of the first color light. The first and second color lights are transmitted through the filter wheel, such that three primary color lights are sequentially projected. The present invention achieves the advantages of reducing cost, simplifying fabricating processes, being available for high working temperature, and further improving image quality without using red phosphor agent or red solid-state light-emitting element.