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Camera calibration is a necessary step in 3-D computer vision, and the accuracy of calibration is largely dependent on the localization precision of the control points of the calibration patterns. However, the sub-pixel localization methods for the commonly used two types of control points, corners of the squares and centers of the circles, are markedly different from each other. In this work, we propose a unified approach to sub-pixel localization for the two types of control points. The approach is derived from theoretical analysis and consists of a closed-form solution, followed by algorithms for practical implementation. Both synthetic datasets and real calibration experiments have been used to test the proposed approach and good results have been obtained. Compared with state-of-the-art techniques, the proposed approach provides a unified and high-precision sub-pixel localization method of the control points for the commonly used calibration patterns.