We herein report a theoretical analysis based on a density functional theory/time-dependent density functional theory (DFT/TDDFT) approach to understand the different phosphorescence efficiencies of a family of cyclometalated platinum(II) complexes: [Pt(NCN)Cl] (1; NCN = 1,3-bis(2-pyridyl)phenyl(-)), [Pt(CNN)Cl] (2; CNN = 6-phenyl-2,2'-bipyridyl(-)), [Pt(CNC)(CNPh)] (3; CNC = 2,6-diphenylpyridyl(2-)), [Pt(R-CNN)Cl] (4; R-CNN = 3-(6'-(2''-naphthyl)-2'-pyridyl)isoquinolinyl(-)), and [Pt(R-CNC)(CNPh)] (5; R-CNC = 2,6-bis(2'-naphthyl)pyridyl(2-)). By considering both the spin-orbit coupling (SOC) and the electronic structures of these complexes at their respective optimized singlet ground (S(0)) and first triplet (T(opt)(1)) excited states, we were able to rationalize the experimental findings that 1) 1 is a strong emitter while its isomer 2 is only weakly emissive in CH(2)Cl(2) solution at room temperature; 2) although the cyclometalated ligand of 3 has a higher ligand-field strength than that of 1, 3 is nonemissive in CH(2)Cl(2) solution at 298 K; and 3) extension of pi conjugation at the lateral aryl rings of the cyclometalated ligands of 2 and 3 to give 4 and 5, respectively, leads to increased emission quantum yields under the same conditions. We found that Jahn-Teller and pseudo-Jahn-Teller effects are operative in complexes 2 and 3, respectively, on going from the optimized S(0) ground state to the optimized T(opt)(1) excited state, and thus lead to large excited-state structural distortions and hence fast nonradiative decay. Furthermore, a strong-field ligand may push the two different occupied d orbitals so far apart that the SOC effect is small and the radiative decay rate is slow. This work is an example of electronic-structure-driven tuning of the phosphorescence efficiency, and the DFT/TDDFT approach is demonstrated to be a versatile tool for the design of phosphorescent materials with target characteristics.