The aim of this study is to discover the organic expression principles of the 20C. Architecture. As a result, first, the organic expression principles was organised in six categories, a. naturalism b. neo-naturalism, c. realism, d. abstractionism, e. regionalism, f. functionalism. Second, among the organic expression principles, the naturalism, the neo-naturalism and the abstractionism were the most dominant organic expression principles of the 20C. Architecture. Third, from 1960, the functionalism have become one of the most dominant organic expression of the 20C. Architecture.