The purpose of this study is to analyze the formative relationship between clothes and hat with the works by Philip Treacy, the designer who raised hats to one of important fashion accessories. The method to analyze the relationship between clothes and hats was the formative analysis by Marian L. Davis and Marilyn R. Delong. The results suggest that clothes and hat had an organic relationship, and a hat style was changed with formative elements of clothes. Clothes and hats by Philip Treacy were analysed in the aspects of Form, Color, Material, and Decoration. As a result, hats by Philip Treacy were mainly designed by the relationship between whole types without a closed line and showed geometric and formative forms, similarly harmonized with clothes. To highlight hats, the achromatic colors such as black and gray were used. Besides clothes and hats were coordinated by the same colors, but contrary colors were used to express a strong image. Felt or straw materials maily used to express a formative and fixed form were well matched with smooth, opaque, and lusterless materials such as wool. Also when transparent materials were used for hats, lace was used for clothes. A hat made of acrylic was matched with clothes made of glossy vinyl coating materials. Decoration was mainly removed but if used, feather decoration was added to clothes.