In order to measure geometrical precision of the thin parts rapidly and accurately, a measurement method based on machine vision was represented. According to the requirement of field of view and precision, hardware and software of the measurement system including the CCD sensor, lens, and the light source were developed. The subpixel edges of the image were extracted after bicubic interpolation subdivision. And the contour of the part is segmented into the arcs and lines by least square fit method with weight function. Then the roundness error and the center distance size error were calculated automatically. The measurement results of gear pump intermediate part show that, this system can used in geometry precision (IT7) measurement of thin parts quickly and accurately.