The effect of light beam polarization on high accuracy measurements of thin wire diameters has been studied. The experimental results are given below in brief. The experimental magnitudes of steel wire diameters vary sinusoidally with the angle between the direction of light vibration and the wire axis. The experimental value obtained with parallel polarization, (i.e., its vibration being parallel to the wire axis) is 0.36 μm smaller than that of the perpendicular, case. The experimental results of steel slit width are contrary to that of steel wires. Based on the experiment, the theoretical analyses of the polarization effect are carried out. Taken into account the influence of light reflection on the wire surface and different phase changes of different vibration directions produced in the reflection, the Fraunhofer diffraction patterns are slightly changed, therefore making the measured values different. According to this idea, the analysis for steel wire shows: the theoretical value of thin wire diameter for light vibration parallel to the wire axis is 0.33μm smaller than that of perpendicular vibration, different from the experimental value 0.36 μm by 0.03 μm. For steel slit width, the measured value with the light vibrations parallel to slit edges is 0.19 μm larger than that of perpendicular case, different from the experimental value 0.21μm by 0.02μm. These differences between theory and experiment are close to or smaller than the error of the measurement system, 0.04 μm. Good coincidencl is achieved.