Columbianadin (CBN, 1), 1-[(8S)-8,9-dihydro-2-oxo-2H-furo[2,3-h]-1-benzopyran-8-yl]-1-methylethyl-[(2Z)-2-methyl-2-butenoic acid]ester is a coumarin-type compound and one of the main bioactive constituents of the underground part of Angelica pubescens Maxim. f. biserrata Shan et Yuan. Although numerous investigations have been undertaken to study the biological activities of CBN, such as analgesic, anti-inflammatory, calcium-channel blocking, and platelet aggregation inhibiting functions, little attention has been paid to its metabolism and/or biotransformation. Biotransformation of CBN by rat liver microsomes in vitro was studied, and thirteen biotransformation products including eight hitherto unknown compounds [columbianadiratimetins A-H (3-10)] and five known compounds [columbianadin oxide (2), (+)-2,3-dihydro-4-hydroxy-2-(1-hydroxy-1-methylethyl)-5-benzofurancarboxaldehyde (11), oroselol (12), columbianetin (13), and vaginol (14)] were produced by liver microsomes from rats pre-treated with sodium phenobarbital. The structures of these compounds were elucidated on the basis of extensive spectroscopic analyses which included IR, UV, EIMS, HRESIMS, 1D NMR and 2D NMR, respectively. The inhibition of CBN and its main biotransformation products on nitric oxide production induced by lipopolysaccharide was assayed in RAW 264.7 cells at concentrations ranging from 10 to 200 μM to evaluate the biological significance of biotransformation.