The paper proposes a method to build standard images by simulating the process of optical imaging and CCD sampling,which provides an objective standard for the evaluation of sub-pixel location operators. Also a novel sub-pixel location operator based on gray area interpolation is proposed,experiments prove that the average accuracy of the arithmetic is about 0.11 pixel for linear edge,and 0.24 pixel for circle edge,which is better than the operators based on gradient centroid and gray moment.