In general, SMA helical springs undergo large deflection under loading during which their mean coil diameter changes considerably. The coil diameter decreases non-linearly with increase in deflection leading to geometrical non-linearity. Whereas during loading and unloading, heating and cooling, change in phase of SMA spring causes change in material properties leading to material non-linearity. This material non-linearity includes variation in modulii and also volume fraction depending on the phases and different zones within the phase. These two non-linearities occur simultaneously which combinedly affects the stiffness of the SMA spring. Models have been developed to predict the behaviour of SMA spring considering both geometric and material non-linearities and experimentally validated.