The influence of metal optical property on the polarization effect in measurements of thin wire diameter with high accuracy has been studied in the present paper. The experimental Values of thin wire diameter obtained from its Fraunhofer diffraction patterns, which are produced by polarized light with the vibrations parallel and perpendicular to the wire axis respectively, is different. The theoretical analysis shows that this difference of wire diameter is approximately proportional to the imaginary part of complex index of metal, and is inde-pent of its real part. Some experimental results agree well with the theory.