A total of 240 Mtwo-NiTi rotary instruments (25/06) were subjected to 0, 2, 3, 5, 10, 15, 20 desinfection and autoclave cycles. The nonautoclaved files (cycle 0) served as control. In two various curved canal models the rotary Mtwo NiTi-instruments were subjected to torsional testing without torque until broken. The time for failure in torsion was measure. The thermical treatment of the NiTi-files invarios cycles influenced their mechanical properties. SEM examinationof fractured surfaces demonstrated the presence of dimples and cones,a typical pattern of dimple rupture deveoped because of ductile failure.