An illumination system includes a first light source, a wavelength conversion device, a first light-splitting element, a light-filtering device, and a light-homogenizing element. The first light source provides a first light beam. The wavelength conversion device includes a single wavelength conversion material configured to convert the first light beam into a conversion light beam. The first light-splitting element is disposed on transmission paths of the first light beam and the conversion light beam. The light-filtering device is disposed on the transmission paths of the first light beam and the conversion light beam. The light-homogenizing element is disposed on the transmission paths of the first light beam and the conversion light beam. When the first light beam is transmitted to the light-homogenizing element, the first light beam is not incident on the wavelength conversion device. A projection apparatus including the illumination system is also provided.