An optical device including a conductive chamber, a first optical module, a second optical module, and a third optical module is provided. The conductive chamber has a light-entrance end. The first optical module, the second optical module, and the third optical module are fixed in the conductive chamber, where the first optical module is adjacent to the light-entrance end and located between the light-entrance end and the second optical module, and the second optical module is located between the first optical module and the third optical module. The conductive chamber, the first optical module, and the second optical module together define a first resonant space. The conductive chamber, the second optical module, and the third optical module together define a second resonant space. An illumination device is also provided.