Abstract: The motion of intervariant intedeces under the action of applied stress in the internally faulted 18R martensite in a Cu-Zn-Al shape memory alloy has been studied. Transmission electron microscopy in situ observations show that the interfaces between 24 martensite variants have different reaction to applied stress. The A/C type and A/B type interfaces have good mobil-ity, the A/D type interface has bad mobiIity, and the different-group-intervariant interfaces are basically immobile.