A process cartridge is detachably mountable to a main assembly of an electrophotographic image forming apparatus. The main assembly includes a motor, a gear receiving a driving force from the motor, a hole defined by twisted surfaces substantially coaxial with the gear and having a grounding contact. The cartridge includes an electrophotographic photosensitive drum, and a projection engageable with the twisted surfaces. The projection is provided at a longitudinal end of the drum. When the gear rotates with the hole and projection engaged with each other, a rotational driving force is transmitted from the gear to the drum through engagement between the hole and the projection. The cartridge also includes a grounding contact electrically connected with the drum, for electrically grounding the drum when the cartridge is mounted to the main assembly, and provided on the projection to be electrically connectable with the main assembly grounding contact.