Abstract: Recently, the on-line measurement technique using CCD image is widely used. This paper presents an on-line image measurement method that can be used to measure the diameter of glass rod. Firstly,the images are obtained by CCD capturing. Secondly, based on the characteristics of the image, the image edge of the glass rod is detected. Then the sub-pixel edge detection method is used to determine the edges accurately. Lastly, the beeline fitting method is used to obtain the diameter of the glass rod. The method not only can substitute traditional manual measurement, but also can control the change of the diameter by machine conveniently. The experiment shows that the method can obtain good result.