A modified method based on mirror rotational symmetry is developed to determine the absolute flatness deviations of optical elements in synchrotron radiation. The novel method is described in terms of functions that are symmetric or antisymmetric with respect to reflections at y axis. Absolute deviations of three flats can be obtained when mirror asymmetric errors are removed by N-position rotation average method. The formulas are derived for measuring the absolute surface error of the rectangular flat, and experiments on high accuracy rectangular flat are performed to verify the method. Compared with the measurement results obtained by Zygo′s three-flat application, our method is calibrated to an accuracy of better than λ/500 (λ=632.8 nm) root mean square (RMS) on height and 0.93 μrad RMS on slope error. The theoretical derivation, experimental results, and error analysis are presented.