American trendy casual styles can be characterized as fitted to body and rendering sexy and fashionable images and now very popular in Korea. This paper aims to identify diffusion channels of Amreican trendy casual style among Korean market and the characteristics of imitation for American trendy casual styles made by Korean casual brands. American trendy casual style was introduced and diffused mainly through 3 channels: American films and TV dramas, on-line communities, on-line shopping mall speciallized for purchasing American apparel brands. Some of American celebrities became fashion ikons and played critical roles in diffusing American trendy styles. After searching websites and fashion magazines and visiting brand stores, several Korean casual brands imitated the styles of American trendy casual brands and the scope of imitation was vast across items. Imitation was found mainly among T-shirts,jacket, jumper, pants, skirt and accessaries. Also imitation was found in almost every aspects of clothes such as shape, sihlouette, color, prints and stitching, belt, and drawstring.