An illumination system including a first excitation light source, a wavelength conversion element, and a scattering element is provided. The first excitation light source emits a first excitation beam. The wavelength conversion element includes a wavelength conversion material. The wavelength conversion element has a first region and a second region. A concentration of the wavelength conversion material in the first region is greater than a concentration of the wavelength conversion material in the second region. The scattering element is disposed on a transmission path of the first excitation beam. A projection apparatus with the illumination system is also provided.