Nickel Titanium alloy (NITINOL) is the sole shape memory alloy in the world which has been used widely in medical applications based on its excellent biocompatible behavior beside its inherent shape memory effect and pseudoelasticity. In the last decade, the porous form of this alloy (PNT) has found its new applications in medicine and especially in orthopedics because of the ability to control its mechanical parameters to reduce stress shielding effect. Moreover, PNT offers a long life implantation and has a structure mimicking bone porous architecture. Despite the use of first PNT implants in orthopedics since more than ten years ago, there has been little effort in understanding and modeling the mechanical behaviors of nickel titanium in porous and more especially in high porous form because of particular characteristics and difficulties exist in modeling this alloy. NITINOL with high porosity may be used for spinal fusion implants for which there has been no modeling approach up to now.Spine fusion surgery by the use of cageis a treatment for patients with degenerative disk diseases. Nowadays, some of these cages are made from high porous nickel titanium alloy.Understanding and modeling the mechanical behaviors of high porous nickel titanium alloy to analyze implants made from which is the main objective of this thesis. At first, study of the biomechanics of the lumbar spine as the target place for implantation of such cages has been conducted. Then different modeling approaches for porous materials have been studied, and a geometrically porous model with tetrakaidecahedron cell structure has been selected for modeling high porous nickel titanium alloy. A new method has been established to produce cell structures with base structure of cubic, tetrahedron and tetrakaidecahedron cells. Then finite element analysis has been used to model a geometrically porous structure mimicking PNT, and the mechanical behaviors of high porous NITINOL have been extracted. In the finite element modeling, due to the highly nonlinear behavior of nickel titanium as well as the modeling scheme selected to implement porosity into the material, three dimensional beam elements was needed to be modeled. Thus a non-linear finite element code for three dimensional beam elements has been developed and verified to be used for PNT structure. Finally, the behaviors of PNT obtained from the nonlinear solution of the problem have been compared with the mechanical parameters of a cancellous bone. The results have been used to predict the behavior of an assembly consists of two vertebrae and two PNT cages between them to show the ability of such cages in bearing biological loads in lumbar spine despite their low elastic modulus. Keywords: Nickel Titanium, Shape Memory Alloy, highly Porous, Implant, Intervertebral Fusion Device, Degenerative Disk Disease, Scaffold