Provided is an electro-photographic image forming apparatus and a method of determining a charging voltage thereof. The electro-photographic image forming apparatus includes: a charge roller for supplying a voltage of a predetermined level to a photoconductor; a transfer roller for transferring a developed image to a printing media by using a developer; a high voltage power supply for supplying a charging voltage at a preset level and a transfer voltage at a preset level to the charge roller and the transfer roller, respectively; and a controller for predicting a charged electric potential deviation based on the transfer voltage to be supplied by the high voltage power supply and for determining a charging voltage level to correct for the predicted charged electric potential deviation. Therefore, the deviation of the charged electric potential caused by the transfer voltage is prevented.