Oxidative damages of lipid and protein in grey mullet (Mugil cephalus) were investigated in vitro by using Fenton reactants (Fe+2SO4/H2O2) to generate hydroxyl radical formation. Thiobarbutiric acid–reactive substances (TBARs, mg maonladehyde/ kg) were used to determine free radicalmediated oxidation of lipids. The oxidative modification of proteins was confirmed by carbonyl formation using with 2,4-dinitrophenylhydrazine (DNPH) and DNPH labeled of oxidized proteins by separated Sodium Dodecyl Sulfate-Polyacrylamide Gel Electrophoresis (SDS-PAGE). Lipids Oxidation of grey mullet with Fe+2/H2O2 gave a noticeable increase in TBARs value during 5h incubation. Protein carbonyl content decreased during the first 3h of incubation time on treatment with free radical generating system and then increased until 5th hours. At 3rd hours of incubation the intensity of the bands near 100 kDa and 150 kDa decreased and the bands above 50 kDa disappeared at 5th hours of incubation after exposure to Fe+2/H2O2. These data suggest that free radical generating system can alter the structure of a protein by leading to protein denaturation and increase in lipid oxidation by leading to development of off- flavors and off-odors during storage, which cause quality loss and limited storage life.