A fabrication process of precisely controlled three-dimensional (3D) microstructures using a maskless gray-scale lithography is described. Multilayered ultraviolet (UV) exposure patterns digitally generated by a commercialized maskless exposure system are superposed on a photoresist-coated substrate layer by layer so as to realize a 3D profile of the UV dose. After a development with an appropriate time, 3D profile of photoresist corresponding to the profile of the UV dose is obtained. Changing the exposure patterns and the exposure time of each exposure makes the precise control of the profile of UV dose possible. The maskless exposure system realizes fabrication of variable three-dimensional patterns at low cost with saving time. As the result of the maskless gray-scale lithography, positive photoresist patterns of spherical and aspherical microlens array of 100 μm in each diameter and 17 μm in height are fabricated. The patterns are transferred into silicon substrates with reactive ion etching (RIE).