We have introduced both electrophoretic and dynamic light scattering to evaluate the polydispersity, nanodispersity, and stability of single-walled carbon nanotubes (SWNTs) in distilled water with surfactants. By controlling the sodium dodecyl sulfate composition and some pretreatment by sonication, we were able to achieve nanodispersion (dispersion into individual nanotubes). The polydispersity was well described by combining both methods. We further showed that the nanodispersion and length distribution observed in the dynamic light scattering spectra were clearly identified by atomic force microscopy. Although surfactants with aliphatic groups can nanodisperse SWNT bundles, the dispersivity and stability depended seriously on the sample preparation process. Our measurements showed that a combination of electrophoretic and dynamic light scattering can provide a convenient and robust means of measuring polydispersity, nanodispersity, and stability of SWNTs in various solutions.