Crystalline morphology of electrospun polymeric nanofibers strongly influences the mechanical properties. In this study, electrospun polycaprolactone nanofibers with smaller diameters that are produced from dilute polymer solutions have a higher degree of molecular orientation, crystallinity, stiffness, and strength, but lower ductility. Nanofiber diameter and the resulting crystalline morphology is influenced by whether complete crystallization of polymer chains took place before or after the electrospinning jet has reached the collector. The former would result in the formation of smaller fibers with fibrillar structure and aligned lamellae, whereas, the latter would result in the formation of a misaligned lamellar structure. (C) 2008 American Institute of Physics.