In order to understand the origin of the anomalous twinning of the {10\\bar12} type, rolled sheets of AZ31 Mg alloy were deformed at room temperature in tension along the rolling direction. An excellent correlation was found between {10\\bar12} twinning tendency and basal dislocation slip activity. Calculation of strain tensor indicated that the diagonal strain components associated with the localized basal slip can be canceled completely by the {10\\bar12} twinning. The results led to the conclusion that anomalous {10\\bar12} twins were formed to accommodate the strain incompatibility caused by localized basal dislocation slip.