Copper nanocrystals with plate-like morphologies and face-centered cubic (fcc) structures have been synthesized in mixed reverse micelles with a large excess of reducing agent. High-resolution transmission electron microscopy images and image simulations show the presence of multiple twin defects parallel to the (111) surface responsible for the forbidden reflections 1/3{422} of the fcc structure. A growth process related to the formation of twin defects is proposed to explain the existence of these morphologies.