An image forming apparatus having an improved power-transmission assembly to drive a developing cartridge mounted in a main body of the apparatus. The apparatus includes a main body, at least one driving coupling unit rotatably located in a side region of the main body, at least one driven coupling unit connected to a rotator inside a developing cartridge, and a coupling holder in which the driving and driven coupling units are connected to each other to enable a power-transmission from the driving coupling unit to the driven coupling unit. The driving coupling unit includes a first spherical portion to come into contact with one side of an inner surface of the coupling holder when received in the coupling holder. The driven coupling unit includes a second spherical portion to come into contact with the other side of the inner surface of the coupling holder when received in the coupling holder.