Abstract: In order to further clarify the effect of oxidation on the protein structure of walnut,walnut protein isolate was studied in this paper,peroxyl radical derived from thermal decomposition of different concentration of 2,2'-azobis(2-amidinopropane)dihydrochloride(AAPH)was selected as representative of lipid peroxidation-derived free radicals. And walnut proteins of oxidative modification by peroxyl radicals were prepared. Effects of peroxyl radical were characterized by protein carbonyl content,free sulphydryl and total sulphydryl content,circular dichroism spectroscopy,intrinsic fluorescence,surface hydrophobicity,particle size distribution and size exclusion chromatogram. The results indicated that,with the increasing of concentration of AAPH,walnut protein carbonyl content increased significantly,free sulfhydryl and total sulfhydryl contents,surface hydrophobicity decreased significantly. The analysis of circular dichroism spectroscopy indicated that exposure of walnut protein to AAPH led to loss of a-helix structure and β-fold structure,and the structure of random coil increased,which indicated that protein secondary structure was damaged by oxidation and converted to disordered structure form ordered structure. The blue shift of wavelength of maximum emission was accompanied by decreased of intrinsic fluorescence intensity,which indicated that peroxyl radical caused molecular aggregation and tryptophan residue was embed. The results of the particle size distribution showed that the soluble aggregate by covalent and non-covalent aggregation could be transformed into an insoluble part. And the results of size exclusion chromatogram showed that the relative molecular mass of walnut protein increased with the concentration of oxidation concentration. Research results in this study indicated that the walnut protein made a significant oxidation in the oxidation system of peroxyl radical and lead to a change in its structure,which would provide theoretical basis for further clarifying walnut protein oxidation mechanism.