The shape memory effect of SMA is due to the thermo elastic martensitic transformation, which is formed by the temperature drop, and it will be reduced if the temperature rises. The difference between the two free energy is the driving force of phase change. The two free equal temperature T0 is called the equilibrium temperature. Only when the temperature is lower than the equilibrium temperature T0, the martensitic transformation can be produced, and the inverse phase transition occurs only when the temperature is higher than the equilibrium temperature T0. In SMA, the martensitic transformation is not only caused by temperature, but also can be caused by stress. The stress induced martensitic transformation is called stress induced martensitic transformation, and the transformation temperature is linear with the stress.