Phosphorescent iridium complexes ranged from far-red to NIR have attracted great attention as oxygen probes or photosensitizers recently. In this work, a far-red phosphorescent iridium complex ((DPQ)₂Ir(acac)) was adopted to prepare biocompatible nanoparticles (Ir-NPs) for both phosphorescence imaging and photodynamic therapy of living cells. The iridium complex was highly sensitive to oxygen in organic solvent, but became less insensitive after being incorporated into NPs, though the particle matrix was highly permeable to oxygen. Moreover, the Ir-NPs exhibited a fast rate of singlet oxygen generation under 660 nm light irradiation. Taking advantage of these Ir-NPs, cellular imaging in the far-red range was realized; meanwhile, in vitro PDT was successfully performed. These results suggested that the Ir-NPs can function as both bio-imaging agents and nano-photosensitizers that work in the far red range.