We report the factors governing zero-magnetic-field splitting (zfs) in the lowest-energy electronically excited triplet (T₁) states of cyclometalated platinum(II) complexes, whose zfs energies between the lowest- (φ₁) and highest-energy spin-sublevels (φ₃) in the T₁ states (ΔE_zfs) are already known: [Pt(bhq)(dpm)] (1Pt), [Pt(thpy)(acac)] (2Pt), [Pt(ppy)(acac)] (3Pt), cis-[Pt(thpy)₂ (4Pt), and cis-[Pt(ppy)₂] (5Pt), where bhq, dpm, thpy, acac, and ppy are benzo[h]quinoline, dipivaloylmethane, 2-(2-thienyl)pyridine, acetylacetone, and 2-phenylpyridine, respectively. As one of the important findings, we show the relationship between the ΔE_zfs and ¹⁹⁵Pt NMR chemical shifts of the five Pt(II) complexes. The implications of the ΔE_zfs values on the emission properties of the Pt(II) complexes in acetonitrile at 293 K are also discussed. In particular, we demonstrate that the radiative rate constants of the Pt(II) complexes correlate with both ΔE_zfs and ¹⁹⁵Pt NMR chemical shifts.