The purpose of this study was to evaluate the protective effects of Chlorella vulgaris extract (CVE) against carbon tetrachloride ($CCl_4$)-induced hepatotoxicity in mice. The mice received silymarin (100 mg/kg), intragastrieally (i.g.) and CVE (50, 100, and 200 mg/kg, i.g.), respectively, every other day, for 4 weeks before $CCl_4$ administration. Twenty-four hr after the administration of $CCl_4$, the serum and liver were analyzed. Our study found that in the CVE groups, aspartate aminotransferase (AST), and alanine aminotransferase (ALT) levels had decreased significantly and the tissue injury was notably diminished compared to the $CCl_4$ group. The antioxidant activities of CVE groups, such as superoxide dismutase (SOD), catalase, and glutathione (GSH), were significantly increased and the activity of nitric oxide synthase (NOS) was remarkably increased in a CVE concentration-dependent manner. In the CVE groups, cytochrome P450 2B1/2B2 (CYP2B1/2) content was decreased. These results indicate that CVE has protective effects against $CCl_4$-induced hepatotoxicity via stimulation of the antioxidant activity and nitric oxide (NO) production, and through inhibition of CYP2B1/2.