A microstructure diffuser includes a light-entering surface, a light-emitting surface, and a plurality of microstructure portions having a first microstructure unit and a second microstructure unit. The first microstructure unit includes a first side surface, a second side surface, a top surface, a first pitch (P1), a second pitch (P2), and a height (H). The second microstructure unit has a curve function shape and is located at the light-emitting surface. The first side surface and the second side surface of the first microstructure unit receive the light beam of the light source to form a first optical path. The top surface of the first microstructure unit receives the light beam of the light source to form a second optical path. The second microstructure unit receives the light beam of the light source to form a third optical path.