Abstract: Because of the drawbacks of the traditional tangent measurement method, a new non-tangent method with the use of digital image processing technology which provides a real time, fast, validity way is presented. It is contains several approaches: image pre-processing, image binaryzation, image segmentation, figure line taking, fitting and smoothing, system calibrating, size calculating etc. A completed system is im-plemented based on this algorithm and shows its feasibility and validity in actual usage.