An illumination system capable of providing at least one light beam to a light valve is provided. The illumination system includes at least one light source, a linearization beam shaper and an optical scanning device. The light source is capable of providing the light beam. The linearization beam shaper is disposed on a transmission path of the light beam and between the light source and the light valve to expand the light beam along a first direction. The optical scanning device is disposed on the transmission path of the light beam and between the linearization beam shaper and the light valve. The optical scanning device is capable of moving for making the light beam scan on the light valve unidirectionally along a second direction or back and forth along the second direction.