Mitochondria play a key role in the energy metabolism of cells, in the process of oxidative stress development as well as in the program of cell death progression. Redox imbalances lead to oxidative stress, changes in mitochondrial membrane potential, affects permeability of the membranes and leads to release of proapoptotic proteins from mitochondrial intermembrane space. Inner organization of mitochondria appears as a key of the accurate operating of the organelle IMM alterations are often associated with structure disorganization and mitochondrial dysfunction, and consequently in numerous pathologies. We investigated the effect of reactive species on shape and structure of mitochondria in Hela cells. ROS production was induced by photoactivated hypericin. Inner structure of mitochondria was investigated by 3D structured-illumination microscopy. By using confocal microscopy and flow cytometry we shown, that increased light dose leads to significant increase of ROS, affects mitochondrial organization and shape a finally leads to cell death. Acknowledgement: This work is supported by 7FP EU CELIM 316310, EU Structural Fund ITMS 26240120040 and APVV-0242-11.