The wafer charging during ion implantation is a serious problem with greater levels of circuit integration in semiconductor device fabrications. Positive charges are accumulated on insulated electrodes or insulators and cause dielectric breakdown if there is no compensation of charging. Ion implantation with negative ions is one of the hopeful techniques to solve this problem. Since an incident ion is a negative charge, an incoming negative charge and an outgoing negative charge of secondary electrons will achieve electrical equilibrium on the insulated electrode. Therefore the charging voltage is extremely low because of this electrical equilibrium. We have measured the charging voltage at several energies of negative ions and also simulated the charging voltage with varying secondary-electron emission factor. We have found that the charging voltage depends on the secondary-electron emission factor.