Reactive oxygen species (ROS) have been associated with acute ethanol-induced liver damage. N-acetylcysteine (NAC) is a glutathione (GSH) precursor and direct antioxidant. In this study, we investigated the effects of NAC on acute ethanol-induced liver damage. Female ICR mice were administered by gavage with a single dose of ethanol (6 g/kg). NAC was administered in two different modes. In mode A, mice were injected with different doses of NAC at 30 min before ethanol. In mode B, mice were injected with different doses of NAC at 4 h after ethanol. Acute ethanol-induced liver damage was estimated by measuring serum alanine aminotransferase (ALT) activity and histopathological changes. Result showed that a single dose of ethanol (6 g/kg) caused a significant increase in serum ALT activity, followed by microvesicular steatosis and necrosis in mouse liver. Pretreatment with NAC significantly protected against acute ethanol-induced liver damage in a dose-independent manner. Correspondingly, pretreatment with NAC significantly attenuated acute ethanol-induced lipid peroxidation and GSH depletion and inhibited hepatic TNF-α mRNA expression. By contrast, post-treatment with NAC aggravated ethanol-induced hepatic lipid peroxidation and worsened acute ethanol-induced liver damage in a dose-dependent manner. Taken together, NAC has a dual effect on acute ethanol-induced liver damage. Pretreatment with NAC prevent from acute ethanol-induced liver damage via counteracting ethanol-induced oxidative stress. When administered after ethanol, NAC might behave as a pro-oxidant and aggravate acute ethanol-induced liver damage.