Diazinon is an organophosphate pesticide commonly used by farmers. The use of diazinon can leave residues on agricultural products. Consumption of these products in the long term will cause health problems through oxidative stress mechanisms. Onion skin (Allium et al.) has a high content of flavonoids, which can neutralize oxidative stress. This study aims to determine the effectiveness of onion peel ethanol extract (OPEE) in reducing the plasma MDA level in diazinon-induced Wistar rats. Method This study is true experimental with a posttest-only control group design in vivo, using 28 samples divided based on a simple random sampling technique into four groups, i.e. the control group (K0), diazinon group (K1), OPEE groups P1 (300 mg/kgBW), P2 (600 mg/kgBW), P3 (900 mg/kgBW), P4 (1200 mg/kgBW), and P5 (2400 mg/kg BW). The examination of plasma MDA levels was carried out using the TBARS method. OPEE at doses of 300 mg/kgBW/day, 600 mg/kgBW/day, 900 mg/kgBW/day, 1200 mg/kgBW/day, and 2400 mg/kgBW/day significantly reduced the plasma MDA levels in diazinon-induced Wistar rats (p<0.05). The plasma MDA level in the OPEE group at a dose of 2400mg/kgBW/day was not significantly different from the control group, indicating that OPEE at a dose of 2400mg/kgBW/day could neutralize the increase of plasma MDA level in diazinon-induced Wistar rat. OPEE is effective in reducing the plasma MDA level in diazinon-induced Wistar rats.