A highly sensitive stopped-flow spectrophotometry kinetic method was proposed forquantification phenoxyl radicals based on their accelerating effect on the oxidation of nicotinamide adenine dinucleotide (NADH). Phenoxyl radicals generated from as low as 1×10-8 mol/L 2,4-DCP can be readily detected with the proposed method and the detecting limit was 2.5×10-9mol/L.