Abstract: A pen self-illuminating when being used is disclosed. The pen includes a barrel with an ink reservoir therein. The ink reservoir has a pair of opposed ears extending from a middle portion thereof and a first spring mounted on the pair of opposed ears. A clip is disposed on the barrel and a switch member is disposed on the clip. The switch member comprises a cover, a mercury battery, a conducting strip, a spacer and a lamp. The cover has an electropad and a terminal thereon. The electropad is connected with one electrode of the mercury battery. The lamp is connected between the electropad and the terminal. The spacer is disposed between the mercury battery and the conducting strip. The conducting strip defines a central opening therein, an elastic trip extending from a periphery defining the opening and a contact strip extending upwardly from a periphery thereof.