The deformation behavior of TiNi shape memory alloy (SMA) under constant and cyclic stress was studied. The stress induced martensitic transformation(SIMT) can't take place immediately when the load is added, so under a constant stress, the total strain will increase gradually. Another special phenomenon called cyclic creep during fatigue was systematically investigated. The maximum stress, loading and unloading rate, and the thermal cycle have effects on the, stress-strain curve shape. All the phenomena has been considered to be caused by the time-dependence of the martensitic transformation.