We report an experimental measurement of the high tensile modulus of single-walled carbon nanotube (SWNT) bundles and SWNT nano-fibers produced by dielectrophoresis. The average tensile modulus of the SWNT nano-fibers is 265 GPa, much higher than the carbon nanotube fibers spun by other techniques. The tensile modulus increases as the diameter of the fiber decreases due to changes in the dieletrophoretic forces that densify the packing of SWNT bundles. Tensile modulus of 643 +/- 86 GPa and 443 +/- 14 GPa were obtained, respectively, from a SWNT bundle of 10 nm diameter and a SWNT nanofiber of diameter 55 nm with a packing density of 69%, respectively. (C) 2009 Published by Elsevier B.V.