In this paper, I examine the modern formation of traditional Korean medicine and discuss the characteristics of the modernization, or modernity, of the medicine. I probe for answers to three questions: first, prior to the twentieth century, what were the main factors that traditional Korean medicine needed to be transformed into a new one? Second, how did four states, the Taehan Empire, colonial Korea, North Korea, and South Korea, treat traditional medicine differently, and why? Third, what are the main characteristics of the modernization of traditional Korean medicine? In examining these questions, I found the following four factors to be important in shaping the modern formation of traditional Korean medicine during the twentieth century: first, the influences of Western science and institutions; second, the rise of nationalism; third, the economics of the state; and fourth, the effectiveness of traditional medicine. Among them, the introduction of Western science and institutions was the most important factor. All the different states in modern Korea realized that Western science and institutions were indispensable for the country to be a powerful nation and to enhance people's welfare. The degree of confidentiality in scientific Western medicine determined the number of traditional medical practitioners and their professional status. The modernization also was greatly affected by modern nationalism, which clashed with Westernization. Many Koreans and the Korean governments regarded the traditional medicine as something culturally valuable to protect from Western culture. Especially, the majority of Koreans who had experienced the cruelty of the Japanese rule under colonization tended to believe that Japan, a foreign ruler, had suppressed traditional Korean medicine as a liquidation policy of Korean culture during the colonial period. This belief contributed greatly to the recovery of the traditional doctors' prestige in South Korea and North Korea after independence. The economic conditions of the country also had an enormous effect on the quantity and quality of traditional medicine in the national medical care. Under poor economic practices, the traditional medicine never was isolated from medical care in the whole country, in spite of the pushing of the ideology of Western medicine. Particularly, since colonial Korea after the 1930s and North Korea after the 1980s were in very poor economic condition, traditional medicine played a more important role than at any other times. In South Korea, since the 1980s, has been making economic success in the course of industrialization toward an affluent society, traditional medicine is being treated as an alternative and complement to the weakness of Western medicine in the area of health improvement and treatment of chronic illness. The deep belief that traditional medicine had excellent effectiveness in this area resulted in this success. Like their ancestors, many Koreans in the twentieth century still had faith in traditional medicine. The traditional medical doctors in Korea succeeded in maintaining their own exclusive professional status, regardless of experiencing various state systems that had different attitudes toward traditional medicine. For the most part of the last one hundred years, Westernization dominated the modern formation of traditional Korean medicine. The modernization of traditional medicine, however, has been overcoming Westernization. Today active communication between Western medicine and traditional medicine is emerging. Western medical doctors, both in South and North Korea, have lessened their prejudiced views of traditional medicine and have more interest than before in the traditional medical treatments such as acupuncture.