Tanshinones are a group of abietane-type norditerpenoid quinone compounds as well as bioactive components in Salvia miltiorrhiza.Copalyl diphosphate synthase of S.miltiorrhiza (SmCPS) is the first key enzyme for tanshinone biosynthesis from the universal diterpene precursor geranylgeranyl diphosphate (GGPP).Hairy roots of S.miltiorrhiza were transformed with A grobacterium rhizogenes carrying RNA interference (RNAi) construct designed to silence SmCPS expression.We examined the expression of SmCPS and the accumulation of tanshinones.Compared with the vector control hairy root lines, in SmCPS-RNAi lines the transcript level of SmCPS was reduced to 26.3%, accordingly the contents of dihydrotanshinone I and cryptotanshinone were reduced to 52.7% and 38.2%, and tanshinone IIA even couldnt be detected.The RNAi-mediated repressed expression of SmCPS caused reduction of tanshinones, which verified SmCPS was the indispensable key enzyme for the biosynthesis of tanshinones in S.miltiorrhiza.