In this work the property of α-lipoic acid (LA) was studied by using pulse radiolysis and laser flash photolysis technology. The characteristics and mechanism of the reactions of LA with ROS (reactive oxygen species) were studied. Transient spectra of LA radicals were observed and the transient species with characteristic absorption spectra were identified. It has been found that LA can effectively quench active oxygen radicals such as ,.,OH, CO,3,.-, SO,4,.-, e,aq,-, Br,2,.-, at the rates of 7.4×10,9, 9.8×10,8, 2.3×10,9, 1.3×10,10, and 2.0×10,9, mol,-1,·s,-1, respectively. The relevant mechanism of the reactions has been discussed. The studies indicate that LA can quench free radicals existing in biological system effectively as a powerful antioxidant.