We calculated temperature dependence of photoluminescence (PL) lifetime of fac tris(2-phenylpyridine) iridium (Ir(ppy)₃) doped in 4, 4'-N, N'-dicarbazole-biphenyl (CBP). The emitting triplet state consists of three zero-field splitting substates. Taking into account non-radiative transitions among these substates, rate equations for the populations of the substates are solved and three PL lifetimes are derived. A good agreement has been obtained for the temperature dependence between the observed lifetime and calculated long lifetime.