An image forming apparatus includes an image forming unit, a power supply configured to receive and convert an alternating current (AC) voltage into at least one direct current (DC) voltage, and output the same, a DC-DC converter configured to convert the at least one DC voltage into a driving voltage at a predetermined level, and a controller configured to be activated by the driving voltage, and control the power supply to turn on the image forming unit by supplying a first DC voltage to the image forming unit, and after the image forming unit has been turned on, supplying a second DC voltage at a higher level than a level of the first DC voltage to the image forming unit.