Abstract: In order to measure the parameters for screw thread with high-precision, a measuring system of thread parameters based on machine vision technology was established. Contour datas of thread in the parallel back light was obtained by thread workpiece rotation and camera following-shot, and the measurements of thread parameters were realized. The measurement range of the system was expanded and the effect of camera calibration errors was reduced. Coordinate conversion equation was derived, and the coordinate system was unified by system calibration. Calculation formulas of thread parameters were derived. Considering the distortion of thread teeth on the projection of screw line when the optical axis of CCD camera and thread axis were vertical, a mathematical compensation model was built. The experiment results show that the uncertainty of system measurement was less than 2m and repeatability was better than 4m. The system can measure the thread parameters.