Abstract: The photo-stabilization effects of combination of 4-hydroxy-2, 2, 6, 6-tetramethylpi-peridine (TMP) with 3-(3, 5-di-tert-butyl-4-hydroxy phenyl) propionic acid tetraester of pentaerithritol (Irganox 1010) in isooctane have been investigated. It is shown from the experimental data that TMP antagonizes with Irganox 1010 during photo-oxidation of isooctane, Irganox 1010 may act as photosensitizer in photo-oxidation of isooctane, in the presence of Irganox 1010 the rate of cumulation of TMPO. is slower and the maximum TMPO.concentration is lower than that in the absence of Irganox 1010, in the case of combination of TMPO. and Irganox 1010 the rate of TMPO. decay is remarkably greater than that in the system containing TMPO. alone. The results observed were discussed in relation to our current understanding of mechanisms of hindered amines and hindered phenolic antioxidants. A new mechanism that the decrease of>NO. concentration was the main reason of antagonism of hindered amines with hindered phenolic antioxidants was suggested.