Increasing evidences have shown the existance of a mitochondrial nitric oxide synthase （mtNOS）, which binds to the matrix face of the mitochondrial inner membrane and produces nitric oxide （NO） through a Ca^2＋ sensitive pathway. Under physiological condition, the NO catalyzed by mtNOS regulates mitochondrial oxygen consumption and transmembrane potential via reversible competition with cytochrome C oxidase. The reaction of NO with superoxide anion, which was produced by mitochondrial respiratory chain, yields peroxynitrite. Peroxynitrite irreversibly modifies susceptible targets in mitochondria and induces oxidative and/or nitrative stress. In addition, NO has also been implicated in the programmed cell death. This article reviews the current understanding of mtNOS＇s role in the regulation of mitochondrial functions