Abstract: The pen structure includes the pen body that has a ball tip for the refill to protrude and the end that is far from the ball tip. The reversible accessory connected the end of the pen includes an extra functional end and a two-way terminal connector that is far from the extra functional end, and a slot that is placed at the end of the pen. When the reversible accessory is connected to the slot by the first terminal connector, it makes the extra functional end shown in the protruding position, and when the reversible accessory is connected to the slot by the second terminal connector, it makes the extra functional end shown in the hidden position. The pen may use its reversible accessory to increase its practicality and extra functions, and when the extra functional end is not used, the unit is shown in the hidden condition, which may keep the appealing appearance of the pen.