A new bipolar charger for nanometer aerosols using soft X-ray photoionization was developed. The concentrations and mobility distributions of the positive and negative ions generated by the X-ray charger were investigated, and then the charging probability of nanoparticles was evaluated experimentally and theoretically. All experimental data of the X-ray charger were compared with those of a 241Am alpha-ray bipolar charger. It was found that the X-ray bipolar charger could produce at least about 3.5 times as high concentration of gaseous ions as the alpha-ray charger and thus could have particles attain the equilibrium bipolar charging state in shorter residence time than the alpha-ray charger. The charging state of particles attained by the X-ray charger was explained by theoretical calculations as well as alpha-ray bipolar charging. It was concluded that the X-ray charger would be a useful instrument for charging high number concentration nanoparticles in the electrical aerosol measurements.