This study investigated a mechanism for controlling the shape of Cu nanocrystals fabricated using the polyol process, which considers the thermodynamic transition from a faceted surface to a rough surface and the growth mechanisms of nanocrystals with faceted or rough surfaces. The faceted surfaces were stable at relatively low temperatures because of the low entropy of perfectly faceted surfaces. Nanocrystals fabricated using a coordinative surfactant stabilized the faceted surface at a higher temperature than those fabricated using a noncoordinative surfactant. The growth rate of the surface under a given driving force was dependent on the surface structure, i.e., faceted or rough, and the growth of a faceted surface was a thermally activated process. Surface twins decreased the activation energy for growth of the faceted surface and resulted in rod- or wire-shaped nanocrystals.