Abstract: In this paper,a diameter measuring technique of slender glass tube based on CCD assembling is presented.Since field of vision is very small,the cameras are calibrated with a grating.During the process of calibration the grating is placed in front of the lens and is imaged on the CCD.By determining the number of pixels of each span,the parameters we need can be calculated exactly.The diameter can be measured by performing some mathematical calculations of the edge position and the parameters obtained by calibration.The errors that affect measuring precision are analyzed.The precision of this system is less than 1μm.The experimetental result shows that the measuring system has quite high stabilization.Of course much improvement can be made to this method.A higher accurate edge-positioning algorithm can be used to improve the precision,and the employment of more advanced image capture card may make the measure result more stable and repid.