Abstract: The influence upon hydroxyl radical of four natural antioxidants:pueraria lobata flavone,extracts of Terminalia Chebula Retz,Rutin,Baicalin is studied by 10-Phenanthroline-Fe2+ oxidative photometric assay,the specific reaction condition where the antioxidants show antioxidant effect is also founded.In the high hydroxyl radical concentration system,these antioxidants may show antioxidant effect,but in the low hydroxyl radical concentration system,they may promote oxidation.The promoting or inhibiting effect of these antioxidants upon oxidation is dependent upon the dosage of themselves.The temperature also produces a great impact upon the effect of antioxidant.