An optical fiber is disclosed, which is comprised of: a core, having a plurality of microstructures formed thereon; and a cladding layer, surrounding the core. In a preferred embodiment, as light is transmitting along the axis of the aforesaid optical fiber and strikes on the plural microstructures, it is scattered and reflected out of the optical fiber through a side wall thereof so as to achieve a side-emitting effect. As the microstructures are formed inside the core of the aforesaid optical fiber, not only they are prevented from being damaged by normal usage, contacting to adhesive directly, but also they can lower the risk of the optical fiber being snapped/deformed while the optical fiber is subjecting to an external force and bended. In addition, by controlling the shape, quantity, size, distribution density and location of the microstructure, the brightness of the side-emitting optical fiber can be adjusted correspondingly.