The antioxidant activities of the various extracts and fractions from the herbs of Artemisia selegensis Turcz (AST) were investigated by in vitro and in vivo assays. FRAP, DPPH and ABTS assays were used to evaluate the antioxidant activities of the extracts/fractions in vitro. The effect of water extract (WE) in reducing oxidative stress in male mice was evaluated. Phenolic acid compounds contribute significantly to the antioxidant activity. From the results of three in vitro antioxidant assays, WE was found to have the highest antioxidant activity, and among the WE subfractions, the water soluble fraction has a significant antioxidant activity. The in vivo antioxidant assay results showed that high doses of WE significantly decrease the MDA level compared to normal diet and D-(+) galactose group (p < 0.05), and the SOD activity of mice given a high dose of WE was the highest. These in vitro and in vivo studies demonstrated that the extracts, especially the WE from AST, have significant antioxidant and free radical scavenging activities. In summary, we propose that AST could be potentially used as a rich source of natural antioxidants.