Mitochondrion is not only the major ATP manufacture center of animal cells, but also plays a key role, as a main switch, in the regulation of apoptos is. Opening of the mitochondrion permeability transition pore (MPTP), the life-or-death switch of cells, causes an increase of the permeability of mitochondrial membrane and the release of several types of apoptogenic factors from the intermembrane space, such as cytochrome c, apoptosis inducing factors (AIFs),procaspases and Ca(2+). These apoptotic factors can either activate caspases, the main member of apoptotic proteins, or destruct independently the intranuclear chromatin, or interact with other Ca(2+) -dependent proteins. These result in structural rupture and dysfunction of cells, and the cells are finally degraded into apoptotic bodies and died. Studies on the regulation mechanisms of mitochondrion in apoptosis will be of great value theoretically as well as potential practical usage in designing drugs for cancer, Parkinson's disease and so on, using mitochondria as targets. To understand the key role of the mitochondrion in the determination of life and death of the cells, recent findings in this field are reviewed in this article.