Culinary-medicinal Royal Sun mushroom, Agaricus brasiliensis (AbS), has traditionally been used for the prevention of a range of diseases, including cancer, hepatitis, atherosclerosis, hypercholesterolemia, diabetes, and dermatitis. The hepatoprotective effect of the fermented mushroom of A. brasiliensis (FMAE) and wild-growing A. brasiliensis (WMAE) were studied in this paper. An in vivo study of carbon tetrachloride (CCl4)-induced antioxidant activity in 2-month-old rats was conducted by examining the levels of activities of alanine aminotransaminase (ALT) and aspartate aminotransaminase (AST) and the antioxidant enzymes, including glutathione peroxidase (GSHPx) and catalase (CAT). Rats were divided into four groups, each containing six rats. The first group served as a control group. The second group was the CCl4 group. Group I and group II were treated orally with distilled water for 14 days respectively. Group III and Group IV were treated orally by WMAE and FMAE at oral doses of 50 mg/kg-day, respectively. Both WMAE and FMAE could reduce CCl4-induced toxicity, particularly hepatotoxicity, by suppressing ALT and AST activities, and increasing antioxidant enzyme activity. The studies demonstrate that both the fermented and wild-growing A. brasiliensis could protect the liver against CCl4-induced oxidative damage in rats.