Strong carbon nanofibers with diameters between 150 nm and 500 nm and lengths of the order of centimeters were realized from electrospun polyacrylonitrile (PAN). Their tensile strength reached a maximum at 1400 degrees C carbonization temperature, while the elastic modulus increased monotonically until 1700 degrees C. For most carbonization temperatures, both properties increased with reduced nanofiber diameter. The tensile strength and the elastic modulus, measured from individual nanofibers carbonized at 1400 degrees C, averaged 3.5 +/- 0.6 GPa and 172 +/- 40 GPa, respectively, while some nanofibers reached 2% ultimate strain and strengths over 4.5 GPa. The average tensile strength and elastic modulus of carbon nanofibers produced at 1400 degrees C were six and three times higher than in previous reports, respectively. These high mechanical property values were achieved for optimum electrospinning parameters yielding strong PAN nanofibers, and optimum stabilization and carbonization temperatures, which resulted in smooth carbon nanofiber surfaces and homogeneous nanofiber cross-sections, as opposed to a previously reported core-shell structure. Turbostratic carbon crystallites with average thickness increasing from 3 to 8 layers between 800 degrees C and 1700 degrees C improved the elastic modulus and the tensile strength but their large size, discontinuous form, and random orientation reduced the tensile strength at carbonization temperatures higher than 1400 degrees C. (C) 2010 Elsevier Ltd. All rights reserved.