A projection apparatus and an illumination system are provided. The illumination system includes a light source emitting a light beam, at least one light-diffusion element disposed on a transmission path of the light beam from the light source and inclined to the transmission path of the light beam from the light source, a wavelength conversion module disposed on a transmission path of the light beam from the at least one light-diffusion element. A long axis direction of a light spot projected on the wavelength conversion module is perpendicular to an extending direction of a boundary between the reference plane and the at least one light-diffusion element. The wavelength conversion module is adapted to convert a first portion of the light beam from the light-diffusion element to a converted light beam. The converted light beam and a second portion of the light beam from the light-diffusion element form a illumination light.