A process cartridge, detachably mountable to a main assembly of an electrophotographic image forming apparatus, includes an electrophotographic photosensitive drum, a developing member for developing an image formed on the drum, first and second end cover portions provided at different ends of the drum, first and second cartridge-bottom-surface portions to be guided by first and second main assembly guide portions, first and second longitudinally-outwardly-drum-projecting portions to be positioned by first and second main assembly positioning portions, respectively, a downstream-projecting third portion to be positioned by a third main assembly positioning portion, and a driving force receiving portion, partly exposed through the first end cover portion between the first and third portions to be positioned, which engages, when the cartridge is mounted to the main assembly, a main assembly driving force transmitting portion to receive a driving force for rotating the developing member from the main assembly driving force transmitting portion.