Abstract: An EHMO calculation has been performed for 2,2,6,6-tetramethyl-4-hydroxypiperidine-l-oxyl(TMHPO). It is shown that the most stable conformation is the chair form with a 12° of “out of plane” angle between the C-N-C plane and the N-O bond, which is consistent with that obtained by X-ray diffraction. The colouration of the nitroxide and the spectroscopic bands of TMHPO in u. v.-visible spectrum may be explained by analyzing the MO levels. The spin density on the nitrogen atom is estimated to be 0.43, from which the ESR hyperfine splitting constant is calculated to be 10.8 G,which is in good accordance with the experimental value.