An illumination device and a lens thereof are provided. The lens includes a surrounding sidewall, a light-incident surface, a light-incident structure, and a light-emitting surface. The surrounding sidewall includes two planar portions opposite to each other and two arc-surface portions opposite to each other to surround a reference axis. The light-incident surface is located at a side of the surrounding sidewall to receive a light emitted from a point light source. The light-incident structure located on the light-incident surface includes a plurality of bar-shaped protrusions. Each bar-shaped protrusion includes a plurality of first light-diffusing surfaces not parallel to the light-incident surface. The light-emitting surface is located at the other side of the surrounding sidewall and is opposite to the light-incident surface. The light-emitting surface is substantially circular. A cross-section area of the lens gradually increases from the light-incident surface to the light-emitting surface.