The g-tensor and relaxation mechanism of ground-state high-spin [Fe(III)OEP]Cl (OEP:octaethylporphyrin) magnetically diluted in diamagnetic NI(IT)OEP single crystals were determined by cw- and pulsed-ESR spectroscopy. From cw-ESR measurements the degree of the rhombicity in fine-structure interaction was determined. It was suggested that the rhombicity is attributable to the pseudo Jahn-teller effect and that the line broadening with increasing temperature is due to the dynamic Jahn-Teller phenomenon among the distorted structures. The large fine-structure constant D was determined to be +7 cm(-1) by measuring the dependences of the spin-lattice relaxation rate on the temperature with the magnetic field perpendicular to the porphyrin plane. The spin-lattice relaxation times highly depend on the direction of the static magnetic field. The orientation dependence was interpreted in terms of the spin sublevel mixing by the spin-orbit interaction.