The effectiveness of low-intensity light for clinical therapies is well demonstrated, however, the principles of these therapies are not well understood. Our aim of this study is the investigation of optical properties of mitochondria because it seems to be the most important cellular organella in terms of the light-cell interaction. We measured the optical absorption characteristics of an isolated mitochondrial suspension in the range of 300 to 900 nm by means of the transmission methods with an integral sphere. Mitochondrial specific absorption coefficients (ml/g/cm-1) were carried out by the transmission spectrum. The specific absorption coefficient at the maximum absorption of cytochrome cγ peaked about 560 ml/g/cm-1 at 418 nm. Many papers described that the effect of the low intensity light to the biological tissue would be drastically changed by the irradiation conditions. We could calculate a deposit energy of irradiated light in a mitochondrion from our results. We think our results may be useful to describe the quantitative explanation in energy balance for the interaction between the light and mitochondria.