In the 1920s-30s, colonial Korea’s electric power industry experienced dramatic growth due to the transformation from a local generating system to a nation-wide grid system consisting of large-scale plants and high-voltage transmission lines. Consequently, the Korean electrical system became more firmly established in the late 1930s as compared to the Japanese system, which was still based on small- and middle-scale power plants. This paper examines colonial Korea’s political-economic context that made this transformation possible while focusing on two large-scale hydropower plants as stepping-stones. The first plant, located at the Pujon River, had a capacity that was 3 times larger than the biggest plant in Japan. The second plant, located at the Changjin River, became the core of the grid system. This transformation was the product of a negotiation between Nippon Chisso Hiryō(Nitchitu) and the Japanese colonial government. Nitchitu, an electrochemical enterprise, needed abundant power resources; the colonial government had to stabilize the public power system. First, upon failing to get sufficient electricity in Japan, Nitchitu tried to secure new power resources in Korea. Nitchitu’s private initiative for expanding its business resulted in the construction of the Pujon plant, which was totally alien to the previous system. Second, the colonial government played an important role in transforming the private plant into an element of the nation-wide grid system. Suffering from the resistance of existing power suppliers and the lack of enough funds, the colonial government chose Nitchitu as an ideal partner. The cooperation between the two made the construction of the Changjin plant possible, which would provide the foundation for the stable grid system.