PURPOSE: To investigate the effects of the underwater Ai-Chi exercise on the joint range of motion, balance and posture. METHODS: Thirty-six college students (30 men, 6 women) were divided into a ‘low physical activity’ groups (Group Ⅰ). and an ‘appropriate physical activity’ groups (Group Ⅱ). The Ai-Chi underwater exercise was conducted three times a week for two weeks for both groups. A goniometer was used to measure the range of motion of the hip joint, and Y-Balance and the posture screen mobile were used to measure the stability of the lower extremities. RESULTS: An evaluation of the range of motion of the hip joint before and after the Ai-Chi exercise showed significant results in the low physical activity group. However, the flexural range showed a significant increase after exercise, but not significant result. In the comparison of the mean increase between groups, only the right hip joint showed a significant difference in both groups. Also, in the comparison of the Y balance test and posture screen test before and after exercise, both groups showed significant. CONCLUSION: The Ai-Chi underwater exercise helped improve the range of motion of the hip joint and the ability to balance. Also It helped improve posture alignment. In addition, although the increase in all physical activity groups lower than the appropriate physical activity groups was greater in all figures, the increase in the number of samples, the extension of the experimental period, and various variables could be obtained.