A process cartridge, and a main assembly of an image forming apparatus to which the cartridge is detachably mountable. The main assembly includes a motor, a driving rotatable member for receiving a driving force from the motor, and a twisted hole substantially coaxial with the driving rotatable member. The process cartridge includes an electrophotographic photosensitive drum, a process device actable on the drum, and a projection provided at a longitudinal end of the drum. When the driving rotatable member rotates with the hole and the projection engaged with each other, a rotational driving force is transmitted from the driving rotatable member to the drum through engagement between the hole and the projection with the projection being pulled into the hole.