This study presents a T-shirt design process that utilizes modernized traditional patterns; in addition, it also includes various modernization techniques for Korean traditional patterns. T-shirts are a representative casual wear item worn in everyday life, especially among young people. The association of traditional elements and public goods such as T-shirts effects the diffusion of the Korean image into global society. Through the whole design process, target groups and design concepts were determined first: collegiate students as well as clean, natural, refined, simple and sporty image, respectively. Subsequently, Dokkaebi, lotus, Dangcho, dragon and Taegeuk patterns were selected as traditional pattern sources. Along with the technique of the division, transformation, and configuration of original patterns, the combining method of geometrical figures, stripe patterns, and check patterns with traditional patterns was helped create modernized traditional motives. Finally, T-shirt designs with a Korean image were developed with the harmonized deployment of modernized motives and color planning. The survey showed that designs had a clean, natural, refined, simple and sporty image that was preferred by collegiate students. This study is relevant in three aspects. First, the design process of modernization was very creative to combine traditional patterns and basic figures. Second, this study expanded the scope of traditional image design to casual wear to promote the Korean image to young people all over the world. Third, the whole design process, from planning to final product development, presented in this study is applicable to other design processes.