Low cycle fatigue behavior of a DZX40M directionally solidified Co-based superalloy has been investigated at room temperature in air under different total strain amplitudes ranging from 2.5 × 10(-3) to 6.0 × 10(-3). The results show that the cyclic hardening of the alloy appears at the first few cycles in the initial hardening stage, and then the saturation stage which is the major part of the whole fatigue life begins. Examination by TEM indicates that the initial hardening of the alloy is caused by the pile-up of the stacking faults at the intersection of stacking fault, while the saturation of the alloy during low cycle fatigue is arising from the transition of the stacking fault on the (111) to the closely spaced stacking fault ribbon on the (111).