Abstract: Objective: The present study investigated the influence of Achillea biebersteinii (Ab), a medicinal herb used widely in Yemeni’s folk medicine as analgesic, antipyretic, against diarrhea and flatulence and for liver diseases, on the liver antioxidant potential of guinea pigs acutely intoxicated with dimethoate pesticide (DM). Materials and Methods: Animals were administered with Ab aqueous extract (50 and 100 mg kg–1 b.wt., orally) or silymarin (100 mg kg–1 b.wt., orally) for 2 weeks followed by single acute DM administration (80 mg kg–1 b.wt., 1/4 of the LD50 orally). Data were analyzed by one-way ANOVA using SPSS. Results: The results showed that the treatment with Ab extract significantly lowered the DM induced serum levels of hepatic marker enzymes (AST, ALT and ALP). Liver histopathology also showed that Ab extract reduced the incidence of lesions including the swelling of cells, lymphocytes infiltration, nucleus fragmentation and condensation and necrosis induced by DM treatment in guinea pigs. Conclusion: The results of this study suggest that Ab aqueous extract could protect the liver against DM-induced oxidative damage.