The purpose of this study is to develop a one-piece dress prototype which has a less impact on breast size. First, I implemented a 3D model with the average of women data done by the 6th Size Korea (2010), and produces avatar A, C, and E with different brassiere cup sizes (A, C, and E). Second, I drafted four types of one-piece dress prototype by measurement avatar A, C, and E. Then I simulated those types of one-piece dress with the application of the 3D Virtual Garment System known as Opti-tex. As a result of virtual garments, all four types of one-piece dress prototype were appropriate on avatar A, but as the breast size increased avatar E, all four types of one-piece dress prototype were inappropriate. Making real garments for three subjects based on one-piece dress prototype I, prototype III and another new prototype and making those subjects wear them, I measured the appearance sensuality evaluation on Likert 5-point scale. The new prototype scored better than the other two protypes I and III. The result does not only increases a sense of satisfaction for women fit for C cup or bigger, but is also available for women fit for A cup or B cup. In addition, it is useful in selecting the brassiere size fit for daily wear.