Ni-Mn-Ga ferromagnetic shape memory alloys (FSMA) are a potential new class of actuator materials able to respond at higher frequencies (at least 300 Hz) with comparable strains (up to 6 %) in a moderate field (below 1 T). Magnitude of the strain depends on the values of several critical material parameters, most importantly the martensitic transformation temperature (TMart), Curie temperature (TC) and saturation magnetization (MS). It is well known that these parameters are strongly dependent on the composition of the alloy. Composition dependence of TMart, TC and MS have been experimentaly explored [3,4]. Therefore, it is possible to compile a more complete, and hence more useful composition map for designing Ni-Mn-Ga FSMAs. Ageing behavior is important in these newly developed FSMAs because ageing can affect the reliability of devices using the alloys. Ni-Mn-Ga FSMAs and Au-Cd alloys have several important common characteristics, including off-stoichiometry alloy composition (designed for operation at ambient temperature) and easy twin boundary motion in the martensite state, thus similar ageing behavior is expected in Ni-Mn-Ga alloys. Ni-Mn-Ga alloys have also demonstrated strong damping due to the motion of twin boundaries. Low-frequency mechanical properties are typically measured using the technique of dynamical mechanical analysis (DMA). In this paper, we present studies of composition design, subtle structure changes associated with ageing, and the temperature dependence of the low-frequency mechanical properties of several Ni-Mn-Ga single crystal alloys.