A new kind of measurement method for ball nut was put forward, which was based on computer vision to measure shape and position errors of ball nut. Firstly, digital image of ball nut was collected by DH-HV3000FC device, and was input into computer by IEEE1394 digital card. Secondly, original gray level image with noise was changed into edge information with single-pixel width after it had been processed by pre-processing, image segmentation and thinning, etc. Thirdly, roundness, cylindricity, ball-way circular runout, concentricity of ball-way axis to the outside cylindrical axis of the ball nut were calculated and measured by compiled program. The value measured was respectively f1=8.34μm，f2=11.94μm，f3=1.76μm，f4=8.87μm. The value measured met the shape and position tolerance requirement of the ball nut. Lastly, the reason of error was analyzed. Theoretical analysis and experimental results showed that this method is feasible and correct.