An image forming apparatus and method including a developing cartridge mounted in a main body of the image forming apparatus to receive a developer with a certain space occupancy of the developer with respect to an inner capacity thereof; a developer supply part connected to the developing cartridge to supply the developer; a sensor for detecting an amount of the developer in the developing cartridge; and a control part for controlling the developer supply part to restrict an amount of the developer for supply to the developing cartridge according to the amount of the developer detected by the sensor, thereby maintaining the space occupancy of the developer in the developing cartridge. Accordingly, the developer supplied to the developing cartridge can be in a certain range of space occupancy during printing, and as a result, density of the developer in a developing area can be uniform.