Low cycle fatigue tests for a hot extruded Nickel base alloy tube material have been performed at room temperature and at 204degreesC. The alloy shows a normal hardening and softening cyclic stress-strain response at room temperature. At 204degreesC, however, the cyclic stress-strain response shows a strain hardening first, followed by a relatively stable stress and finally a secondary cyclic strain hardening. This stable stress disappears with increasing strain amplitude. The mechanisms of the secondary cyclic strain hardening have also been investigated by transmission electron microscopy (TEM). Besides dislocation multiplication, interactions between stacking faults and moving dislocations and between interstitial atoms and moving dislocations could also contributed to this secondary cyclic strain hardening. The formation of micro-twins during cyclic loading at 204degreesC and its influence on the cyclic stress-strain response were also discussed. (C) 2004 Kluwer Academic Publishers.