A unipolar charger containing multiple discharging wires in a tube (inner diameter: 50mm) was developed and tested in order to increase the aerosol flow rate and the charging efficiency of nanoparticles. Four gold wires of 25m in diameter and 15mm in length were used as the discharging electrodes to generate positive ions (Ni) from 2.72108 ions/cc to 3.87109 ions/cc in concentration at the discharging voltage of+4.0+10KV. Monodisperse NaCl particles of 1050nm in diameter were used to test the charging efficiency and the particle loss of charged particles with different aerosol flow rates, corona voltages and sheath flow rates. The sheath air near the tube wall was found to increase the extrinsic charging efficiency, and the highest efficiency was obtained at+6.0KV discharging voltage, 10L/min aerosol flow rate and 9L/min sheath flow rate. The extrinsic charging efficiency increased from 10.6% to 74.2% when the particle diameter was increased from 10 to 50nm. The TDMA (tandem differential mobility analyzer) method was used to determine the charge distribution and the mean charge per particle and it was found that the Fuchs charging theory corrected for the extrinsic charging efficiency matched with the experimental data very well.