Oxidatively shortened single-walled and multiwalled carbon nanotubes were characterized by using flow field-flow fractionation (FIFFF) under normal and steric modes, respectively. Narrow size fractions were collected from FlFFF separations. The carbon nanotubes in each fraction were further characterized by using scanning and transmission electron microscopy. FlFFF separates carbon nanotubes principally on the basis of length, leading to fractions with relatively uniform lengths. The relationship between nanotube length and FlFFF elution volume is discussed.