Mitochondria are involved not only in energy metabolism but also in free radical metabolism. Superoxide anion can be generated through a way of electron leak of respiratory chain and the reactive oxygen species (ROS) can be formed in the further reactions of O₂^-_· in mitochondria. The role of mitochondria in anti-oxidant functions and cell apoptosis is discussed in terms of electron leak of respiratory chain, uncoupling of oxidative phosphorelation, mitochondrial pore, Box- or/and PTP-mediated release of cytochrome c from mitochondria and so on. The signaling act of ROS is emphasized in the regulation of cell apoptosis.