An illumination apparatus includes a reflective shell, a front cover, a light-emitting device, and a plurality of optical micro-structures. The front cover is connected to the reflective shell. A containing space is formed between the reflective shell and the front cover. The light-emitting device is disposed in the containing space and is configured to emit a light beam. The optical micro-structures are disposed on at least one of the reflective shell and the front cover. Each of the optical micro-structures is a multilayer stair-structure. The optical micro-structures diffuse the light beam by refraction and reflection.