American trendy casual style can be characterized as fitted to body and rendering sexy and fashionable images and now very popular in Korea. This paper aims to identify how American trendy casual styles have been introduced and diffused among Korean young people and also compare the similarities and differences between American and Korean styles. American trendy casual style was introduced and diffused through 4 channels: people who have living experiences in America, media exposure by American films, TV dramas and fashion magazines, on-line network using internet cafe, club and blog, use of 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. Photos of those American celebrities and street fashion of Korean young women were collected through internet search from Sep. 2006 to Aug. 2007. 6 styles including leggings, mini skirt, short dress, skim jeans, pants with long boots, big bag and long necklace were selected as representing American trendy casual styles. Silhouettes and overall styling were very similar in all 6 items. But Differences were found in terms of rotor, amount of details, item coordination, matching shoes and overall images.