In this paper the mechanism of {10 13} twinning was studied through the application of the atomic group rotation model. The atomic displacement vectors of some selected atoms were calculated and the general formulas for calculating the displacement vectors of the twinning atoms were obtained; the atomic group rotation mechanism was then proposed. It was found that two types of alternately distributed atomic groups exist in {10 13} twinning as in {10 11} twinning. The atomic groups rotate by an angle of 6.3°, and a relative displacement magnitude of 0.539 a is produced. The larger relative displacement magnitude produced during twinning may be the reason that {10 13} twinning occurs more rarely than {10 11} twinning.