Cyclometalated Ir(iii) complexes, with a long triplet state lifetime and good photophysical properties, are good candidates for simultaneous imaging and photodynamic therapy (PDT). Herein, we synthesize a cyclometalated Ir(iii) complex, Ir(tiq)2ppy, whose triplet excited state lifetime is 2.9 μs and singlet oxygen generation quantum yield is approximately 100% (compared to tetraphenylporphyrin). Ir(tiq)2ppy nanoparticles (Ir(tiq)2ppy NPs) are prepared to achieve water solubility and mitochondria-targeting ability by co-precipitating with polystyrene grafted with carboxyl-terminated poly(ethylene glycol) (PS-PEG). Ir(tiq)2ppy NPs have higher PDT efficiency than Ir(tiq)2ppy at concentrations as low as 1.6 μg mL-1 for MCF-7 breast cancer cells under white light irradiation at quite low light intensity (5 mW cm-2). Besides, it is worth noting that the emission quenching of Ir(tiq)2ppy in aqueous solution has been conquered by using Ir(tiq)2ppy NPs, thus the distribution of the therapeutic agents in mitochondria can be tracked by confocal laser scanning microscopy (CLSM). The mechanism of killing cancer cells under irradiation is investigated, and the results indicate that cell death is caused by mitochondria-mediated apoptosis, which is induced by the ROS generated under light irradiation.