The fatigue properties of thermomechanically processed Nitinol wires were investigated with comparisons between shape memory (SM) and superelastic (SE) behaviors. Three compositions of Nitinol wires [Ni49.5Ti50.5 (SM), Ni50.6Ti49.4 (SE) and Ni50.8Ti49.2 (SE)] were characterized under rotary bending tests for zero mean strain conditions. Two compositions of Nitinol tubings [Ni49.5Ti50.5 (SM), Ni50.8Ti49.2 (SE)] were tested with diamond shaped fatigue specimens for non-zero mean conditions. The resultant experimental data were interpreted with computational analysis for strain based fatigue analysis. The shapes of the strain-life curves change drastically as the composition changes. These results will be discussed in terms of microplasticity and phase transformations.