An illumination system is provided, which provides a plurality of light beams to a light valve. The illumination system includes a plurality of light sources, a polynomial lens and an optical scanning element. The light sources are capable of emitting the light beams. The polynomial lens is disposed on light paths of the light beams and located between the light sources and the light valve. The polynomial lens shapes the light beams into a plurality of rectangular light beams. The optical scanning element is disposed on light paths of the rectangular light beams and located between the polynomial lens and the light valve. The optical scanning element is capable of moving for scanning the rectangular light beams on the light valve unidircetionally or back and forth along a direction, and the rectangular light beams partially overlap with each other on the light valve.