Abstract: In order to solve the problem that the hole diameter of the hard disk is easy to be affected by the local strong reflection and the high reflective surface, a method for measuring the diameter of circular parts to removal the noise based on the fractional differential was presented.The fractional-order differential algorithm was used to process the images of the hard disk aperture images with strong reflection and high reflection, so as to eliminate the influence of irrelevant information in strong reflection surface around circular hole on the extraction of the edge of circular hole.Experiments were carried out to compare with the Prewitt, Soble and Laplacian operators.It is proved that the fractional differential can effectively reduce the amount of image information to be analyzed and achieve a better visual effect.Moreover the Canny edge detection was performed for the circular hole image after processed by fractional-order differential algorithm to extract the available circular hole edge.Finally, the least square method was used to measure the hole size.Experimental results show that, compared with other algorithms, in the case of accurately preserving the edge information of the round hole and effectively suppressing the surrounding noise, the error can be controlled around 0.05 mm, and the the measurement accuracy is guaranteed.