To investigate the chemical constituents from the branch of Fraxinus sieboldiana, and evaluate their antioxidative activity. The chemical constituents were isolated and purified by chromatographic techniques over silica gel, macroporous adsorbent resin, Sephadex LH-20, and preparative HPLC. Structures of the compounds were identified by spectroscopic methods. The antioxidant activity was evaluated by Fe(+2)-cystine induced rat liver microsomal lipid peroxidation. Eight coumarins were obtained and their structures were elucidated as esculin (1) , esculetin (2), fraxin (3), fraxetin (4), 6, 7-di-O-beta-D-glucopyranosylesculetin (5), scopoletin (6), cleomiscosin D (7) and cleomiscosin B (8). At a concentration of 10(-6) mol x L(-1), compound 4 showed antioxidative activity inhibiting Fe(+2)-cystine induced rat liver microsomal lipid peroxidation with inhibitory rate of 60%. Compounds 5, 7 and 8 were obtained from the genus Fraxinus for the first time. Compound 4 showed remarkable antioxidative activity, which was higher than that of VE (35%).