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Results are presented on the cyclic deformation of single crystal NiTi containing Ti₃Ni₄ precipitates of various sizes. Mechanical cycling experiments reveal that the cyclic degradation resistance of NiTi is strongly dependent on crystallographic orientation. Under compression, orientations approaching the [100] pole of the stereographic triangle possess the highest fatigue resistance. Orientations approaching the [111] pole of the stereographic triangle demonstrate the lowest fatigue resistance. Aging to produce small coherent Ti₃Ni₄ precipitates (10 nm) improves the fatigue resistance of NiTi compared to the other heat treatments (solutionized or overaged) for nearly all orientations. NiTi with 10 nm Ti₃Ni₄ precipitates consistently showed stabilized martensite due to mechanical cycling, and an absence of dislocation activity. Samples with large incoherent Ti₃Ni₄ precipitates (500 nm) consistently showed significant …