Charging characteristics of carbon nanofibers (CNFs) with a 241-Am charge neutralizer were investigated using diameter- and length-controlled CNF particles, which were generated by a floating catalyst CVD method. The neutral fraction and charge distribution of CNF measured by aerosol techniques suggested that the fraction of neutral particles is much lower than that predicted by the conventional charging theory for spherical particles and that there exist a large number of multiply charged particles in charge equilibrium. Furthermore, it was found that the charge equivalent diameter that is twice the one proposed in a previous work gives a good prediction for the charge distribution of CNF with a diameter smaller than 20 nm and aspect ratio between 5 and 40.