The atomic mechanism, based on the previously proposed ``inhomogeneous'' shear, leading to the formation of twinning and antiphase boundaries in TiNi with the CsCl‐type structure is described. The twinning mechanism described herein explains the electrical resistivity anomaly due to ``incomplete'' thermal cyclings observed previously in TiNi. This explanation is in keeping, in a qualitative manner, with the ``memory'' effects observed in relation to the electrical resistivity anomaly.