The purpose of this study is to provide objective data on the functionality of sports-bras for women with 75A small breasts, which is the average breast size of Korean women, and to help recognize the importance of wearing sports-bras while exercising. The contra group was composed of 4 brands of sports-bras, which were preferred by 200 unmarried women between 25~34 years of age who had experience of wearing sports-bras, and also were thought to be an adequate comparison. The contra sports-bra models were comprised of a compression-type sports-bra, a compression-type sports-bra with shoulder straps, a lined-type sports-bra with cutting lines on the cups, and a mad-type sports-bra with two molded cups. The results of this study are as follows. Breast movement during exercise was more prominent on the lateral sides of the breasts than the medial sides, and the movement increased as the exercise intensity increased. While running, breast movement was most prominent when the front foot touched the ground after being airborne, and least prominent just before taking the leap with one foot on the ground. On comparison of the study and control group sports-bras, contra B showed the most prominent breast movement on exercise, and control A showed the least movement.