In order to improve the precision of vision inspection system for micro accessory, edge detection technique of micro accessory was studied. Firstly, the imaging features of the micro accessory inspection system were analyzed. Fraunhofer diffraction happens, because the light wave is restricted by microscope. Secondly, optical characteristics of micro images were analyzed. The gray curve of the micro image edge does not change with the different inspected accessories. The threshold function of the edge detection was put forward according to the Analysis of dividing rule diffraction fringe. Finally, the threshold function of the edge detection was applied to gauge block and pendulum block. Experimental results indicate that the maximum error of the gauge block inspection is 0.22μm and the maximum error of the pendulum block inspection is 0.28μm. The measuring precision of figures is up to sub-pixel and the measuring precision of micro accessories is up to submicron. It can satisfy the system requirements of the micro accessory inspection. .