Melt-blown nonwovens are widely used in filter materials, fiber diameters and distributions play an important role in affecting the filtration efficiency of melt-blown material. The traditional method of microfiber diameter testing mainly relies on manual measurement, which is time-consuming and inaccurate. A large number of experiments show that the images quality of melt-blown material captured by optical microscope is not appropriate for automatic image processing. These images were not only vague, but are very difficult to get the clear edges of fibers by Edge Detection, such as LAPLACE, LOG and CANNY, etc. In such circumstances, based on scanning electron microscope (SEM) images, it proposes a new image processing method for measuring microfibers diameters. The new method consists of many steps: edge detection, image segmentation, thinning, lines linking and rectangle matching, which is successful in making fast, accurate automated measurements of microfiber diameters. The validation experiments show that the microfibers diameters and its distribution of melt-blown material can be measured accurately by the proposed method.