Abstract: Fe(Ⅲ) in H₂SO₄ medium cancatalyze the decotorizing reaction which results from the oxidation of Methylred by hydrogen peroxide. In fixed time and tempera ture, the difference of the spectrome tric values(A-A) between the catalytic rectionand the non-catalytic reaction have satisfactory linsear relation with the concentration of Fe(Ⅲ) wihtin the range of 0～0.4g/25ml. The new method has been applied to the determin tion of micro amount iron in biological samples. The sensitivity of the method is 2.0×10^-11g/ml