This paper investigates the relationship between the functional fatigue level and the frequency response behaviour of NiTi Shape Memory Alloys. The specimens, which are SMAs with 0.7-millimeter diameter, were subjected to a constant stress under different number of thermal cycles, leading to different fatigue levels. The fatigue SMAs were tested in open-loop experiments with step inputs and sinusoidal inputs, and their frequency responses were evaluated as the magnitude and phase of the wires' displacement, with respect to the input voltage signal. The results of the experimental study showed that the dc gains of the specimens, subjected to ranges of stresses from 76 to 204 MPa, decreased as the fatigue levels of SMAs increased. Also, the differences between frequency responses of fatigue wires and those of healthy wire were noticeable over the frequency range up to the bandwidth of the wire, especially when the wires were loaded with lower stresses.