Ni-rich NiTi shape memory alloys undergo multiple-step transformations after thermomechanical treatment. Microstructural changes induced by thermomechanical treatment affect the superelastic behavior of shape memory wires. On the other hand multiple-step transformation may lead to new applications if the strains of stress induced transformation are adjustable for each step. Therefore this study focuses on the stress assisted and stress free aging effect on super-elastic properties of Ni-rich NiTi wires while undergo multiple-step transformation. The evaluation of transition temperatures has been conducted by differential scanning calorimetry (DSC). Electrical resistance measurements were performed during tensile tests. The experimental results show that aging of solution annealed and water quenched Ni_50.9Ti_49.1 wires at 450°C for 45 minute lead in three step transformations in both stress free and stress assisted aging. DSC results indicate that the aging treatment under different applied stresses can not affect the characteristic temperatures. However stress assisted aging result in obvious difference in super-elastic properties of the wires. In the aging under stress, results of tensile tests show two distinct plateaus in different stress levels. Results of electrical resistance measurements show that these two plateaus are related to transformation of B2 and R to B19´.