Abstract: A new method for the determination of hydroxyl free radical (·OH) produced by fenton reaction was proposed.Reaction of hydroxyl radical with acridine red resulted in decrease of fluorescence intensity and the production of free hydroxyl radical was indirectly determined.Experiments verified that decrease of fluorescence intensity was related to the amounts of reactants of acridine red,FeSO4 and H2O2.The antioxidant thiourea has a scavenging effect on the·OH redical.The proposed method has been applied to the determination of antioxidant ability of some pungent foods.