Abstract: The high order perturbation formulas of EPR parameters for d⁸ configuration in trigonal crystal field are derived.In our formulas, the contributions due to all excited states including singlets are considered. g_⊥ > g_|| or g_⊥ < g_|| and D > 0 or D< 0 are explained as due to the expanding or contracting of ligands along C₃-axis. The formula obtained is applied to deal with EPR parameters of NiCl₂ and NiSiF₆·6H₂0 crystals.The agreement of theoretical calculation with experiment shows that the theoretical calculated values are good.

Key words: zero-field split, zero-field split, trigonal crysfal field, d⁸ configuration