Problem statement: Drought stress as a major adverse factor can lower leaf water potential, leading to reduced turgor and some other responses and ultimately lower crop productivity in arid and semi arid zones. Sunflower is one of the main oil seed crops in Iran, where drought stress is the most limiting factor. Drought stress tolerance requires the activation of complex metabolic activities including antioxidative pathways, especially Activated Oxygen Species (AOS) and scavenging systems within the cells which can contribute to continued growth under drought stress. Approach: To evaluate the effect of limited irrigation systems and potassium fertilizer on grain yield, antioxidant enzymes and lipid peroxidation biomarker (MDA), the crop was sown in the Research Farm of College of Agriculture, Islamic Azad University, Karaj Branch in 2009. The experimental treatments were arranged as split plots based on a Randomized Complete Block Design with three replications. The main plots were allocated to three different irrigation regimes. The irrigation regimes comprised of: Full Irrigation (IR 1 ), Moderate drought stress (IR 5 ) and Severe drought stress (IR 2 ). The subplots were allocated to four potassium chemical fertilizer (Potassium nitrate) consisting of K 1 = 25, K 2 = 50, K 3 = 75 and K 4 = 100% recommended. Results: Plants under drought stress and potassium levels showed a significant increase and decrease, respectively, in SOD, CAT and GPX activity and MDA in compared to control plants. In this context, plants with higher levels of potassium showed higher resistance to drought stress conditions and higher yield and dry matter allocation to grain filling process i.e. harvest index. Results of this study suggested that drought stress leads to production of oxygen radicals, which results in increased lipid peroxidation (MDA biomarker) and oxidative stress in the plant. Conclusion: The scavenging of AOS by the scavenging system especially by SOD, CAT and GPX was done well and damage to membranes or MDA was controlled at higher levels of potassium.