This study is to know the influences on the modern fashion by Claire McCardell who was a major role on setting the ready-to-wear industry in 1930-1960 in America. To do the theoretical study, it progressed its historical background for Europe and America, and then studied the work for Claire McCardell`s lifetime, her fashion philosophy, and her fashion features. Through the world war Ⅰ and world war Ⅱ, American fashion industry took a rapid growth. Claire McCardell raised sportswear to be the best item between the women`s wear and set the foundation for the modern fashion in the American fashion history. It can be summarized by next three prat of the Claire McCardell`s influences on the modern fashion. First, She changed the concept of functionalism of sportswear to be the day-wear`s. Second, She played a very important part in promoting the industrialization of the modern fashion by developing one piece items. It mixed and matched the items, and it was suitable for the modern fashion industrial system which formed a line collected by diverse items. Third, she led the western fashion trend by presenting a new dress model proper for the American mental and lifestyle which was differentiated from the European`s. Claire McCardell did her best effort to fill women`s inner expression with their eager and their necessity in modern life style rather than creating the fashion for a fashion.