Mitochondria, one of cellular organelles, are involved in ATP production, maintenance of calcium homeostasis, cell growth, differentiation, and apoptosis. Mitochondrial oxidative stress induced by reactive oxygen species (ROS) damage physiological characteristics, and cause apoptosis as the oxidative stress of the cell exceeds the threshold. Therefore, inducing mitochondrial oxidative stress is efficient target for proceeding photodynamic therapy (PDT). Recently, iridium(III) complexes have attracted attention as photosensitizer for photodynamic therapy (PDT) because of their characteristics including high ROS generation efficiency, high stability in physiological condition. However, they have a limitation of low absorption coefficient in the visible region, which let high-energy irradiation (>35 J/cm2) necessary for iridium(III) based PDT. Herein, we report mitochondria-localized Iridium(III) photosensitizer, Ir-OA, incorporating acedan dye as energy donor. The acedan dye has strong absorbance and fluorescence which is well matched to the absorbance of iridium complexes. As the Ir-OA is irradiated, the absorbed energy by acedan donor is transferred to the iridium(III) with high efficiency (>98%). As a result, the enhanced amount of triplet exciton of Ir-OA improved ROS generation, which cause effective cell death with ultra-low energy irradiation (<0.08 J/cm2). In addition, the Ir-OA is capable to image morphology, polarity, and viscosity change while applying oxidative stress to mitochondria. The photophysical properties of Ir-OA is variable depending on the polarity and viscosity change, which enable to tracking mitochondrial oxidative stress by ratiometric imaging and fluorescence lifetime imaging microscopy (FLIM).