An easy method of measurement of the zeta potentials of sub-50-nm polymeric nanoparticles is suggested. Although zeta potential measurements of nanoparticles or emulsions above 50 nm have been successfully carried out, zeta potentials of emulsions or nanoparticles below 50 nm could not be precisely measured in the region of extremely low conductivity by conventional electrophoresis with a He-Ne laser. However, zeta potentials of sub-50-nm nanoparticles were measured in the region of thin electrical double layers by addition of sodium chloride and zeta potentials in the condition without sodium chloride could be predicted by extrapolation of the measured potentials. The electrophoretic mobility of 150-nm nanoparticles stabilized with sodium dodecylsulfate was the same as that calculated from extrapolation of the measured ones. The zeta potentials of sub-50-nm nanoparticles stabilized with sodium dodecylsulfate could be calculated by the same procedure.