A pen includes a housing, and an ink reservoir to be received in the housing. The ink reservoir includes a cartridge received in the housing and for receiving ink, a barrel attaching to front of the cartridge, and a nib attached to the barrel, for extending out through the front opening of the housing. The provision of the barrel between the cartridge and the nib allows the ink reservoir to be accommodated into various housings of different lengths and of different structures. The barrel includes a peripheral rib extended from a peripheral swelling for engaging with a spring member which may bias the barrel against the housing.