Two trimeric proanthocyanidins, cinnamtannin B-1 (1) and cinnamtannin D-1 (2), have been isolated from the bark of Cinnamomum cassia along with the known tetramer parameritannin A-1 (3) and a previously unreported tetramer, named cassiatannin A (4). The structures of 1-4 were elucidated on the basis of 1D and 2D NMR, MS, and CD analyses and compared to the reported data. Proanthocyanidins (1-4) possess significant in vitro inhibitory activity against cyclooxygenase-2 (COX-2) at micromolar concentrations.