Nanocrystalline materials are single or multi-phase polycrystalline solids with a grain size of a few nanometers (1nm=10 ?9 m), typically less than 100 nm. These materials have properties vastly different from the conventional coarse-grained material which makes them a perfect candidate as raw materials used in vehicle and aero industries. In such materials a grain boundary is the interface between two grains . These boundaries are the cause of decrease in the electrical and thermal conductivity and elastic constant of the material. In nanocrystalline materials, the existence of very fine grain sizes and consequent high density of interfaces are the reason to a variety of properties that are different and often considerably improved in comparison with those of conventional coarse-grained materials. These may include increased strength/hardness, enhanced diffusivity, improved ductility, toughness, reduced density, reduced elastic modulus, higher electrical resistivity, increased specific heat, higher coefficient of thermal expansion, lower thermal conductivity, and superior soft magnetic properties. In this study, the mechanical properties of iron single crystal with body-centered cubic structure and nanocrystalline iron were investigated by using molecular dynamics method. In order to obtain results in a reasonable period of time with a large number of atoms, a molecular dynamic package has been used, which is executable on the Graphical Processing Units. Embedded-Atom Method (EAM) interatomic potential models were utilized as a typical force field for dense materials to simulate the properties of iron. Elastic constants of iron single crystal at different temperatures are obtained employing Nose-Hoover integration method. The elastic constants in non-zero temperature were calculated using fluctuation formulas which are functions of potential energy, as well as the momentum of the particles in the system. The results have shown good agreement with the existing experimental results. Next the Young's modulus of polycrystalline iron was predicted based on results obtained from iron single crystal model. In addition, the coefficients of linear thermal expansion in different temperatures were also calculated for iron single crystal. Afterward, computational investigations on nanoindentation of bcc iron using a spherical indenter were performed. Also additional indentation simulations were performed on the grain boundaries and the calculated Young's modulus of both grain and grain boundaries were in reasonable agreement with the experimental data. The model of nanocrystalline was generated using the Voronoi tessellation method. The model consists of three grains and about 100,000 atoms. In this model, the grains having a random crystallographic orientation are arbitrarily placed within a cube. With Voronoi tesselation, one can make a random nanocrystalline sample with a grain boundary structure similar to what is expected in polycrystalline materials. Obtained results implied that the strength of grain boundary is weaker than that of grain regarding the results of the indentation process. The effect of grain size on the elastic properties of nanocrystalline alpha-iron is also reported. Softening of the elastic properties is observed for grain sizes ranging from 8 nm down to 6 nm. The decrease in the Young’s modulus with decreasing grain size is in agreement with experimental data and matches an analytical model based on the rule of mixtures for composite materials. Keywords: Nanocrystalline material, Molecular Dynamics, Iron, GPGPU, Elastic Modulus, Nanoindentation.