In this study, we present the design of an automatic ring expander with stepped cap structure. If the ring inner diameter does not match the one's diameter, the ring should be expanded in order to match the diameter. Conventional ring expanders have been manually operated by inserting the ring into a cone-shaped cap and expanding the diameter of the cap. Because of the cone-shaped caps, however, the top and the bottom of the ring could not be equally expanded or the ring can be damaged due to the manual operations. Therefore, we introduced new design for automatic ring expander with the stepped cap and the ball screw connected to the stepping motor. The diameter of the ring is increased by pushing a wedge-shaped rod into the cap according to the rotation of the motor. Using the finite element analysis, the final design of the ring expansion mechanism was validated in the ring expansion force requirement.