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Bulletin, The purpose of this study was to verify the validity of joint motion measurement in lower limbs during underwater dolphin kicking using inertial measurement unit (IMU) sensors. One female competitive swimmer participated to this study, and she performed a 20 m underwater dolphin kick swimming at her maximum effort. To evaluate the validity of the methodology using IMU sensors, we used both IMU sensors and 3D motion capture system to analyze the hip and knee joint motions during one kick cycle. From the results of the cross-correlation analysis, the maximum r-values in hip flexion/extension angle was 0.94 and the maximum r-values in knee flexion/extension angle was 0.94, while the maximum r-value in hip internal/external rotation angle was 0.37. Therefore, it was shown that the validity of measurement for hip rotational motion during underwater dolphin kicking was low. Furthermore, the ranges of differences in peak hip and knee flexion/ extension angles through one kick cycle were 1.7―13.3 degrees, while the similarities in these angle changes were very high. From these results, it was clarified that it can be applied to evaluate the changes of hip and knee joint angles during underwater dolphin kicking while it was difficult to compare the absolute joint angles.