Dibutyl—o—phthalate (DBP) was irradiated with ,γ, ray sealed in air or nitrogen at ambient temperture. The radiolysis products were analyzed by means of GC, GC/MS and PGC/MS,etc. Serious radiation degradation of DBP was showed. Not only general products such as hydrogen, methane, carbon dioxide and ethylene etc were examined,but also hydroxy or carbonyl compounds such as water, formic acid,allyl acohol, butyl alcohol,butyl aldehyde and phthalic anhydride etc were detected,two novel products 3—methyl benzofran and 3—methyl—dihydro—benzofran were found also. Remarkable effects of oxygen in ambience on formation of products hydrogen and carbon dioxide, but little effect on methane, were observed. The content of these three gaseous products all increased with dose,there was basiclly a linear relationship between the concentration of methane and dose. The radical reaction mechanism of dibutyl—o—phthalate radiation degradation was discussed.