The aim of this study was to determine the oxidation of proteins, measuring the protein carbonyl levels (PCO) as biomarkers of oxidative stress, and to answer whether protective effect of green tea catechin, EGCG, can reduce the protein damage initiated by free radicals in guinea pigs' liver tissues under 50 Hz: 12 kV/m E Field exposure. Guinea pigs, weighted 250-300g, were used in the study. Protein carbonyl levels (PCO) were measured spectrophotometrically, by Levine method with slight modifications. Mann-Whitney U test was applied for statistical analysis. As a result, both 50 Hz 12 kV/m E Field exposure and EGCG-administrated (intraperitoneally) groups' liver protein carbonyl levels were found non-significantly decreased than control groups. However 50 Hz 12 kV/m E Field exposure + EGCG-administrated group's liver protein carbonyl levels were found significantly decreased. In the view of these results, it was concluded that E-Field exposure may inhibit the formation oxidized protein, reducing the protective effect of EGCG.