A proanthocyanidin trimer (1), two tetramers (2 and 3) and a pentamer (4) have been isolated from the root bark of Cinnamomum sieboldii MEISNER (Lauraceae). The structures of these compounds were established by acid-catalyzed thiolytic degradation, in conjunction with proton and carbon-13 nuclear magnetic resonance analyses. In addition, the presence of (-)-epicatechin (13), (+)-catechin (14) and known proanthocyanidins B-1 (5), B-2 (6) and B-5 (7), trimers (8 and 9), tetramers (10 and 11) and a pentamer (12) in this plant was demonstrated.