A comprehensive and concrete exploration into the deactivation mechanisms of luminescent materials is imperative, with the improvement of simulating and computing technology. In this study, an integrated calculation scheme is employed on five Ir(III) complexes for thorough investigation of their photophysical properties, including radiative ( kr) and nonradiative ( knr) decay rates. As a most famous Ir(III) complex with superior quantum efficiency, fac-Ir(ppy)3 herein serves as a reference relative to the other four β-diketonate complexes. Both temperature-independent and temperature-dependent knr are taken into account quantitatively for the first time, to unearth the role of different ancillary ligands in the determination of luminescent properties. Since the validated calculations of kr for the five complexes are of the same order of magnitude, the nonemissive peculiarity of 4 is caused by large knr. The newly designed compound 5, which simply has two more -CH2 groups than 4 in the ancillary ligand, further manifests that the reason for large knr in molecule 4 should be attributed to the ligand resonance caused by great π conjugation.