Erigeron breviscapus is a well-known traditional Chinese herbal medicine. In this study, on-line HPLC-ABTS/DPPH assay coupled with MS detection were applied to screen and identify the free radical scavengers in 70% methanol extracts of E. breviscapus. Using on-line HPLC-ABTS-MS and HPLC-DPPH-MS assay, 13 radical scavengers (including 4-O-caffeoylquinic acid (4-CQA) (1), 9-caffeoyl-2,7-anhydro-2-octulosonic acid (9-COA) (2), 3-caffeoyl-2,7-anhydro-3- deoxy-2-octulopyranosonic acid (3-CDOA) (3), erigeside I (4), quercetin-3-O-glucuronide (5), eriodictyol-7-O-glucuronide (6), scutellarin (7), 1,4-di-O-caffeoylquinic acid (1,4-di-CQA) (8), 3,5-di-CQA (9), 1-malonyl-3,5-di-CQA (10), erigoster B (11), 4,5-di-CQA (12) and 4,9-di-CDOA (13)) and 9 radical scavengers (including 1, 4, 7, 8, 9, 10, 11, 12 and 13) were discovered, respectively. Furthermore, the anti-oxidative activities of 4 compounds, including 7, 9, 11 and 12 were evaluated. Reverse anti-oxidative activity order of scutellarin and 3,5-di-CQA was observed in on-line HPLC-ABTS assay and on-line HPLC-DPPH assay. To validate their anti-oxidative activities, the off-line ABTS and DPPH assays were performed. Given sufficient reaction time, 3,5-di-CQA showed higher activity than scutellarin, which was consistent with the order obtained in on-line HPLC-ABTS assay. These results revealed that on-line HPLC-ABTS assay is a more sensitive method for screening and determining free radical scavengers, especially more suitable for those compounds with slower reaction kinetics. © 2012 Informa UK, Ltd.