The rhizome of Alisma orientale Juzep (Alismataceae) has been used as a crude drug for diabetes, edema, inflammation and urinary disturbances in oriental medicine. Recent animal studies have shown that the extract of Alisma orientale rhizome (AOR) can potently lower high levels of serum lipids and improve insulin resistance, which are usually detected in patients and animals with non-alcoholic fatty liver disease. So, we studied the antioxidative effects of AOR extracts and fraction in vitro and their protective effects against acute hepatotoxicity induced by $CCl_4$ in vivo.. We then investigated the effects of each fraction on hepatotoxicity induced by tert-butyl hydroperoxide (t-BHP). DAOR (dichloromethane fraction of the Alisma orientale rhizome) scavenged free radicals and superoxide anions. DAOR protected against $CCl_4$ induced hepatotoxicity. DAOR had hepatoprotective and antioxidative effects against t-BHP-induced hepatotoxicity in HepG2 cells and in rats.