A portable printer for printing on a roll of paper or label stock is provided having automatic print alignment with the width of the roll. The portable printer has a housing having a compartment for receiving the roll, a cover to access the roll, and a centering mechanism for the roll. The centering mechanism has two rotatable spindle members in the compartment engageable with the opposing ends of the roll's tubular core, and a pair of racks each coupled to one of the spindle members, and to each other by a gear, to enable each of the spindle members to move in opposite directions with respect to a center between the spindle members. The position of centering mechanism is optically, magnetically, or electro-mechanically encoded and a sensor reads the encoded position of the centering mechanism. A controller automatically aligns printing with respect to the roll's width in accordance with the encoded position read by the sensor, thereby preventing printing outside the width of the paper from the roll. The centering mechanism may be locked when the cover is closed to prevent movement of the gear, and the spindle members and racks coupled thereto. A removable RF communication module may be provided in the printer to enable communication with a host terminal or computer system.