Friction stir welding using a bobbin tool (BT-FSW) is a kind of friction stir welding (C-FSW) process in which a lower shoulder is added to the general friction stir welding tool. It is known as the useful method to reduce the Z-axis load required during the tool insertion and traveling for FSW. In addition, it can eliminate the backing plate, and actively respond to part deformation. For this reason, research on the application of BT-FSW is expanding from low melting temperature materials such as aluminum and magnesium to high melting temperature materials. However, no relevant studies on BT-FSW can be found in Korea. This study aimed to present tool design guidelines specifically for BT-FSW. Summarized information on variables to be considered when designing the BT-FSW tool has been described, such as pin and shoulder diameter, shoulder gap, pin and shoulder feature. The design of the tool is a significant step in set-up the BT-FSW process, and should be considered according to the material characteristics (viscosity, plasticity…) and thickness. The pin diameter that is similar to the thickness of material, and the shoulder diameter that is 2 to 3 times wider than the pin diameter were recommended. The shoulder gap, the distance between the upper and lower shoulder, is generally machined to match the material's thickness or fabricated to be shallower than. Since the pin shape directly affects the vertical and horizontal direction movement into the stir zone, the pins with 3 to 4 flat faces or threaded are more practical than the cylindrical pin shape.