128×128 diffractive microlens arrays have been designed by considering the relative optical and processing parameters for 3～5 μm wavelength with a microlens diameter of 100 μm. The lens F number and array pitch are f/1.5 and 100 μm, respectively. The diffractive microlens arrays have been fabricated on the surface of Si substrates by successive photolithography and Ar+ ion-beam-etching technique. The practical processes and fabrication method are discussed. The optical characteristics and measurement of the surface relief structure are presented.