Liver fibrosis is a common scarring response to all forms of chronic liver injury and is always associated with inflammation that contributes to fibrogenesis. Although a variety of cell populations infiltrate the liver during inflammation, it is generically clear that CD8 T lymphocytes promote while natural killer (NK) cells inhibit liver fibrosis. However, the role of invariant natural killer T (iNKT) cells, which are abundant in the liver, in hepatic fibrogenesis, remains obscure. Here we show that iNKT‐deficient mice are more susceptible to carbon tetrachloride (CCl₄)‐induced acute liver injury and inflammation. The protective effect of naturally activated iNKT in this model is likely mediated via suppression of the proinflammatory effect of activated hepatic stellate cells. Interestingly, strong activation of iNKT through injection of iNKT activator α‐galactosylceramide (α‐GalCer) accelerates CCl₄‐induced acute liver injury and fibrosis. In contrast, chronic CCl₄ administration induces a similar degree of liver injury in iNKT‐deficient and wild‐type mice, and only a slightly higher grade of liver fibrosis in iNKT‐deficient mice than wild‐type mice 2 weeks but not 4 weeks after CCl₄ injection, although iNKT cells are able to kill activated stellate cells. An insignificant role of iNKT in chronic liver injury and fibrosis may be attributable to hepatic iNKT cell depletion. Finally, chronic α‐GalCer treatment had little effect on liver injury and fibrosis, which is attributable to iNKT tolerance after α‐GalCer injection. Conclusion: Natural activation of hepatic iNKT cells inhibits, whereas strong activation of iNKT cells by α‐GalCer accelerates CCl₄‐induced acute liver injury, inflammation, and fibrosis. During chronic liver injury, hepatic iNKT cells are depleted and play a role in inhibiting liver fibrosis in the early stage but not the late stage of fibrosis. (HEPATOLOGY 2009.)