This article is a historical account of the production of opium and its monopoly in early colonial Taiwan. It analyses how the Japanese colonial government successfully turned the “opium problem” in Taiwan into a well-controlled substance on the island and details the mechanisms that led it to be a profitable monopoly business. Taking a material-oriented and STS approach, this research focuses on the practical details of the production of opium pastes. It examines the interplay between the many varieties of opium, local workers and connoisseurs, Japanese technicians and authorities within the laboratory and factory in which opium was studied, analyzed and made. The study of opium monopoly in colonial Taiwan (1895-1945) extends our understanding of opium smoking by showing the impact laboratory and factory, as well as modern measurement and standardization, had on the making and mass-producing of opium pastes in early colonial period, and reminds readers of the power and socioeconomic structure that was behind and sustained these technoscientific practices.