This Study is about the tradition of Organic Medievalism expressed in Modern Architecture. The concept of Medievalism is an attitude to revive the social and physical settings on the Middle Ages. The Organic Medievalism in Modern Architecture was to be explained as the two tendencies; one was toward the rational structural logic of the medieval architecture being re-interpreted into the Modern Architecture and the other was toward the organic and fantastic shape of the Modern Architecture, closely connected with nature-friendly and organic shape of medieval city and buildings. The structural logic of the medieval architecture became systematized by the French architects, Viollet-le-Duc and Auguste Choisy. The principle has been applied to the design of the Art Nouveau, Louis Kahn and so-called the High-Tech Architecture creating the aesthetics of its own by exposing the building structure. The other was used as the elements to represent the characteristics of the Art Nouveau architects, and then served as the background of the creation of the avant-garde architecture of Germany. The ideal has been serving as the great idealistic fundamentals by the so-called Archigram and Deconstruction at the late half of the 20th century.