A ground-state triplet diphenylmethylene derivative, 2,2′,3,3′,5,5′,6,6′- octamethyldiphenylmethylene (OMDPM) is a model for chemically stable organic high spin molecules. The electronic structure and the stability of OMDPM have been investigated by electron spin resonance, and the molecular conformation responsible for this stability has been shown by AM1 UHF MO calculations and LCAO MO calculations for zero-field splitting parameters. OMDPM was generated by photolysis of the corresponding diazo precursor in organic solvents or single crystals of the precursor at cryogenic temperatures. Based on the simulation of the fine-structure spectra from random orientation, the zero-field splitting parameters have been determined. The single-crystal experiment showed that OMDPM survived up to 250K. The AM1 UHF MO calculations and the LCAO MO calculations made clear the reason of this stability. Both calculations indicated that the phenyl rings of OMDPM were perpendicular to each other and that the reactive center was protected by the eight methyl groups.