An illumination system configured to provide an illumination beam is provided and includes a light-source module and a first and a second wavelength conversion devices. The light-source module provides a first and a second excitation beams. The first wavelength conversion device is disposed on a path of the first excitation beam and has first regions and at least one first boundary disposed between every two adjacent first regions. The second wavelength conversion device is disposed on a path of the second excitation beam and has second regions and at least one second boundary disposed between every two adjacent second regions. The second regions correspond to the first regions. A time point when the first boundary gets into the path of the first excitation beam is different from a time point when the second boundary gets into the path of the second excitation beam. A projection apparatus is also provided.