This article illuminates how international politics in the Cold War era influenced the rice breeding program in South Korea in the 1960-70s. The “Green Revolution” in South Korea in the 1970s was the outcome of amalgamation of two different traditions in agricultural science: Japanese colonial legacy and postwar American influence. From the early twentieth century, Japanese agricultural institutions in Korean Peninsula replaced indigenous cultivars and cultivation techniques with Japanese ones based on modern agricultural science. The hegemony of Japanese agronomy in Korea remained undiminished up to the late 1960s, primarily because Japan was the only center of scientific research on “Japonica” rice varieties. Meanwhile, as South Korea had gradually been incorporated into American hegemony since its independence in 1945, American agronomy began to exert influence to South Korea. In the late 1950s, in particular, the “Atoms for Peace” project significantly affected agricultural science, with the promise of mutation breeding program. The boom of mutation breeding did not last long in Korea, but American influence gained a new momentum in the 1960s with the initiation of the Green Revolution in rice, which was based on the classical way of cross-breeding. In the mid-1960s, South Korean agronomists visited the International Rice Research Institute (IRRI) in the Philippines and succeeded in hybridization of IRRI’s Indica rice with Japonica varieties. The new hybrid rice, Tongil [reunification], was highly productive and disease-resistant, and soon endorsed by South Korean government as the spearhead of the Green Revolution. With massive mobilization in rural communities, Tongil contributed in doubling domestic rice production in the 1970s. Tongil rice disappeared in the early 1980s, after its poor crops were coincided with the collapse of the authoritarian Park Chung Hee regime in 1979. However, Tongil is still remembered by South Korean agronomists as the most important achievement in the history of South Korean agronomy, especially in that they took advantage of the American Cold War strategy to deviate from the Japanese colonial legacy.