Abstract: The electron spin resonance spectra of 4-oxo-2,2,6,6-telramethy 1-piperidine-1-oxyl in twenty-six solvents of different polarity were deter mined.It was found that the hyperfinc splitting constant A_Nincreased with the increase of the polarity of the solvent while the g value showed a tendency of slight decrease.There was a linear correlation of the A_Nvalue with Reichardt E_T and Kosower Z,but no such correlation was found with e or μ Since E_T,Zare microscopic solvent polarity parameters for model reactions whereas ε,μ macroscopic solvent polarity parameters for nonmodel reactions,A_Ncan thus be used as a microscopic solvent polarity parameter for nonmodel reactions.The bonding molecular orbital energy and antibonding molecular orbital energy of the hcteronuclear diatomic three electron bond in nitroxide were calculated by HMOmethod,using the semi-empirical value of h_iand k_iiparameters for heteroatoms given by Streitwieser,and it was shown that the delocalization energy was within the range of 0.4-0.5β,comparable to that for butadiene (HMOmethod).The results could account for the stability of nitroxide radical and also the solvent effect upon the distribution of π-electron density.