An illumination system including a first excitation light source, a wavelength conversion wheel, and a filter wheel is provided. The first excitation light source is configured to emit a first excitation beam. The wavelength conversion wheel includes a wavelength conversion region and a first optical region. The wavelength conversion region and the first optical region alternately enter a transmission path of the first excitation beam. The first excitation beam is converted into a conversion beam by the wavelength conversion region when the wavelength conversion region enters the transmission path of the first excitation beam. The filter wheel includes a first region, a second region, and a blocking region. An angle covered by the second region in a circumferential direction is smaller than an angle covered by the first optical region. A projection apparatus is also provided.