As the traditional instruments for web thickness measurement is inefficient for the micro drills with an outer diameter of less than 0.30mm,a novel method on the basis of image processing is proposed.In order to im- prove the instrument accuracy,a comparison is made among three subpixel edge detection algorithms,and the gray moment algorithm is selected as the subpixel edge detection algorithm by comprehensively considering the error and time consumption.Then,two micro drills respectively with the diameters of 0.40mm and 0.25mm are used to veri- fy the proposed method.The results show that，for the micro drill with a diameter of 0.40mm,the proposed method on the basis of image processing presents a repeatability of 1μm,which is higher than that of the traditional contact measuring method; and that the repeatability keeps 1μm even for the micro drill with a diameter of 0.25mm,which means that the proposed method is effective in measuring the web thickness of micro drill with a diameter being less than 0.30mm.