The part alignment is an important step for automated optical inspection of printed circuit board (PCB). Because conventional image recognition algorithm could only be implemented at pixel accuracy, it is not satisfied with the requirements of PCB inspection. Through the use of geometric symmetry of a circle, a robust subpixel algorithm for the center of circle detecting is presented. By the proposed method the measurement error might be less than 0.01 pixel for noise free images. In the meantime, experiments show that the algorithm has a relatively small bias in the presence of noise.