In order to accurately get the geometric parameters of an object, we designed a bilateral telecentric optical system to collect images of the object and measured the size of object by digital image processing techniques. We used Zemax to optimize the system, analyzed the aberration of the image and the transfer function of the system. The system had a long working distance which was greater than 74 mm. The field-of-view was 80 mm. The largest distortion was less than 0.11 percent and MTF was larger than 0.3 at 190 lp/mm for full field region of the CCD sensor. We analyzed the stability of the magnification and measurement deviation, and measured the small parts. The deviation was in the required range.