Oxidative stress is an important molecular mechanism for kidney injury in mercury (Hg) poisoning. Usually, oxidative stress was measured by measuring the levels reactive oxygen species (ROS) and antioxidant enzymatic activity. In this present study, we proposed a new marker for oxidative stress in kidney toxicity induced by Hg. The new marker is the peroxidative index (PI). In this experiment, a kidney sample was taken from male rats (Rattus novergicus). Samples then homogenized and divided into five groups with; T1 served as control which contains kidney homogenate only; T2 which contains kidney homogenate+0.1 mg/l of mercury chloride (HgCl); T3 which contains kidney homogenate+1 mg/l of HgCl; T4 which contains kidney homogenate+10 mg/l of HgCl; and T5 which contains kidney homogenate+100 mg/l of HgCl. After treatment, kidney catalase (CAT) and peroxidases (Pox) activity, PI, hydrogen peroxide (H2O2) and PC level were estimated. The results revealed that Hg level is strong negatively correlated with both CAT and Pox activities, and strongly positively correlated with PI. Also, the results revealed that PI is strongly positively correlated with PI with the presence of Hg in different concentration of Hg in kidney cells. In conclusion, PI might be a useful marker for oxidative stress in kidney damage induced by Hg. For our knowledge, the proposed mechanism according to our results is Hg inhibited antioxidant enzymatic activity and increase ROS in the kidney. Thus, induced oxidative stress which promotes a further reaction to damage protein and resulted in kidney damage. Keywords: Kidney, Mercury, Oxidative Stress, Peroxidative Index.