A process cartridge of an image forming apparatus including a development cartridge including, a development cartridge housing with a toner inlet port therein, and a toner inlet port cover movably formed at the development cartridge housing, to open and close the toner inlet port, having a rib at a leading end thereof and a locking recess at a lateral surface thereof; and a toner cartridge including, a toner cartridge housing, detachably engaged with the development cartridge, having toner provided therein, a toner supply part provided with a toner supply port and a locking protrusion to contact the rib of the toner inlet port cover, and a locker to lock in the locking recess of the toner inlet port cover. As the toner cartridge is engaged with the development cartridge, the locking protrusion of the toner supply part contacts the rib of the toner inlet port and the locker is locked in the locking recess of the toner inlet port so that the toner inlet port cover moves in engagement and disengagement directions with respect to the toner cartridge to open and close the toner inlet port of the development cartridge.