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Abstract: A star centroiding error is a critical error in determining the accuracy of star sensors. As an important component of the systematic centroiding errors, the S-curve error is known for its well-regulated sine shape in a period of a pixel. On the basis of the frequency domain analysis, the analytical expression of an S-curve error is given in this paper. Accordingly, the sources resulting in the S-curve error are comprehensively investigated in this study along with their specific role in determining the characteristics of the S-curve error. Experiments are carried out using a real star sensor with the help of a precise turning table and a starlight simulator. The S-curve error data are collected, and a corresponding correction model in the form of a sine function is developed on the basis of the analytical expression. The correction of the S-curve error is well verified by the additional collected test data in the entire field of view (FOV) with an improvement of at least 35.7% in the star centroiding accuracy. Consequently, the calibration accuracy of the star sensor is also improved by 31.7% after the S-curve error correction. Further experiments are performed to investigate the S-curve errors of stars having saturated pixel energy, and the results of these experiments agree with those of the simulations to a large extent. All the analytical and experimental results indicate that the S-curve error has significant regularity and can be corrected using the proposed method. [Copyright &y& Elsevier]