The intrinsic viscosity of five well-characterized polystyrene-block-polyisoprene nanofiber fractions in THF was determined using viscometers of different shear rates. The values increased pronouncedly with decreasing shear rates, suggesting the ready alignment of the nanofibers along the shearing direction. The intrinsic viscosity data of the longer fractions were treated by the Yamakawa-Fujii-Yoshizaki (YFY) theory for wormlike chains to yield reasonable persistence length and hydrodynamic diameter. The validity of the YFY theory should allow the use of intrinsic viscosity in the future for evaluating the molar mass of block copolymer nanofibers.