In this study, we discuss the changes of the macroscopic material properties of a Ni-Ti alloy in relation to its pseudoelastic microstructure formation. Ni-Ti alloy is a material which is often used as a stent, and its crystal structure is the face-centered cubic (FCC) austenite above 700-800K and body-centered cubic (BCC) martensite below that temperature level. In the austenite phase region at temperatures above the shape recovery temperature, Ni-Ti alloys undergo a stress-induced transformation to martensitic phase when an external force is applied and immediately return to the original austenite phase when the external force is removed. This gives Ni-Ti alloys a rubber-like elasticity without plastic deformation. When the FCC austenite structure of Ni-Ti alloy changes to a BCC martensite, the single crystal grain in a polycrystalline aggregate accommodates some stable phases due to the lattice misfit between crystal structures. The BCC phase, which is called pseudoelastic phase, is known to exhibit stress relaxation due to the change of crystal structure when the external deformation is applied. In this study, the emphasizes are placed on the mechanism of pseudoelastic phase transformation caused by the change of lattice structure during external deformation. In order to reproduce the behavior of stent, we propose a mathematical model to predict the formation of pseudoelastic phases in crystal grains of a Ni-Ti alloy. The phase-field model equipped with the elastic energy [1] is introduced to realize the morphology formation of pseudoelastic phases in a crystal grain. We conduct a series of numerical simulations to reproduce the deformation-induced nucleation and growth of pseudoelastic phases of Ni-Ti alloy. The crystal orientations of matrices are also considered in the simulation so that the pseudoelastic phase transformation in the polycrystalline aggregates can be reproduced. Then, the changes of the macroscopic material properties are discussed in relation to the pseudoelastic microstructure formation. REFERENCES [1] M. Muramatsu, K. Yashiro, T. Kawada and K. Terada, Simulation of Ferroelastic Phase Formation Using Phase-field Model, International Journal of Mechanical Sciences, Vols. 146-147, pp. 462-474, 2018.