Cannabis sativa produces novel secondary metabolites called cannabinoids. Among them, Δ1-tetrahydrocannabinol (THC), the psychoactive component of this plant, has attracted a great deal of attention, because this cannabinoid is shown to exert a variety of therapeutic activities such as the relief of nausea caused by cancer chemotherapy. To reveal the mechanism of THC formation in Cannabis sativa, we attempted biosynthetic studies. Consequently, we established that Δ1-tetrahydrocannabinolic acid (THCA), the precursor of THC, is derived from cannabigerolic acid and that this reaction is catalyzed by the novel oxidocyclase, THCA synthase. Here, we show the biochemical and structural properties of THCA synthase.