Based on the principle of laser diffraction, a method of measuring the wire diameter which is less than 12μm is introduced. Using the least square method fitting the diffraction intensity of the central bright stripe, the wire diameter can be obtained in high-precision. Carbon fiber and Xinjiang long-staple cotton are used as samples, and three lasers with different wavelength are used to carry out the experiment. It is realized that accurate experimental results can also be obtained by using low precision teaching light intensity detectors and smart phone light sensors.