A new design of a unipolar aerosol charger has been developed. The charger consists of a radioactive source placed between two screen electrodes enclosed by a Plexiglass tube. The electric field in the charger is aligned with aerosol flow. The new charger is capable of charging ultrafine aerosols efficiently and with low particle losses. The charger was evaluated thoroughly both theoretically and experimentally. The basic equations for the charging process in this charger were numerically solved for monodisperse, ultrafine aerosols under various operating conditions. From the calculation results, the particle charge level and loss rate within the charger were found to depend on two dimensionless parameters defined in the paper. In the experiment, the particle charge level and penetration rate were measured for particle sizes of 10-30 nm, operating voltages of 3-9 kV, flow rates of carrier gas of 2-5.21 min-1, and pressures of 0.46 and 1 atm. The experimental results which were obtained under similar conditions estimated by the dimensionless parameters were found to agree with the theoretical predictions. The charger gave a unit charge to 50% of the 10 nm diameter particles with 20% particle losses. The charger was found to work also under low pressure as a high-efficiency charger.