We prepared a polymeric microlens array (MLA) using ultraviolet (UV) light to cure photosensitive monomers through a photomask. After a short-time UV exposure, the uncured monomers experience a process of partial wetting and self-development on the surface of cured monomers. As a result, a geometric relief with a lens character is generated. Depending on the pattern of the photomask, either a convex or concave MLA can be fabricated. The mechanism of forming the MLA is explained and the concept is proved experimentally. Owing to the merits of simple fabrication, good flexibility, and high optical performance, the MLA has potential applications in light diffusers, fiber/organic light-emitting diode couplers, biomedical imaging, and displays.