Six protoberberine alkaloids were isolated from the chloroform layer of the rhizome of Coptis japonica Makino (Ranunculaceae). The structures of the isolated compounds were determined to be 6-([1,3]dioxolo[4,5-g]isoquinoline-5-carbonyl)-2,3-dimethoxy-benzoic acid methyl ester (1), oxyberberine (2),