The mechanism of X-ray irradiation on the function and structure of mitochondria was studied by analysis of mitochondrial swelling-shrinkage, oxidative phosphorylation, Fe ions induced swelling, and Fe ions induced lipid peroxidation. Mitochondria were isolated from the livers of mice at 3, 6, 12, 24 and 48 hours after whole body irradiated 500r with X-ray, and from non-irradiated mice. The results of the experiment are as follow: 1. The activity of oxidative phosphorylation of mitochondria was reduced by 50 per cent after irradiation for 6 hours and returned to normal after 24 hours. 2. Respiratory control of mitochondria was decreased by 40 per cent after irradiation for 6 hours and remained at this low level for 48 hours. 3. The degree of swelling-shrinkage of mitochondria (about 20 per cent changes in volume) measured by 90° light scattering was reduced to some extent during 6 to 12 hours after irradiation and then recovered to control level after 24 hours. 4. The large amount of mitochondria to show the certain extent of 90° light scattering was required at 6 hours after X-irradiation and returned to normal after 48 hours. 5. Swelling, measured by the absorbancy at 520mμ, and lipid peroxidation of mitochondria induced by Fe ions were increased in there initial velocity and in there extent by X-irradiation. These increments were maximum at 12 hours. 6. These results suggest that the increment of peroxidizable lipid of mitochondria is a fact for the uncoupling of oxidative phosphorylation with X-irradiation, The possible role of uncoupling of oxidative phosphorylation by X-irradiation has been discussed.