An inertial measurement unit (IMU) was developed by programming in an Arduino environment to obtain breathing motion data from nine semi-professional swimmers in a university swimming team while they swam four 50m laps using the butterfly stroke. The results indicated that the proposed device can help coaches and researchers distinguish breathing and non-breathing processes and effectively read the pitch angle and time information of breathing motions during the butterfly stroke. Furthermore, a hierarchical linear model (HLM) approach was employed to examine the interaction effects of the individual variables of the swimmers and their breathing patterns on their swimming efficiency. This analysis showed that significant interaction effects exist between age and average breathing time, significantly influencing swimming efficiency. It also indicated that significant interaction effects exist between gender and the number of breaths taken and between gender and average maximum breathing angle. Lastly, significant interaction effects exist between the body mass index (BMI) of the swimmers and the number of breaths taken and between BMI and average maximum breathing angle. These results demonstrate the efficacy of the proposed IMU, which could be effectively applied to help coaches and researchers analyze and enhance swimmers’ performance.