Recent experiments on polycrystalline materials show that nanocrystalline materials have a strong dependency to the strain rate and grain size in contrast to the microcrystalline materials. In this study, mechanical properties of polycrystalline materials in micro and nano level were studied and a unified notation for them was presented. To completely understand the rate-dependent stress-strain behavior and size-dependency of polycrystalline materials, a dislocation density based model was presented that can predict the experimentally observed stress-strain relations for these materials. In nanocrystalline materials, crystalline and grain-boundary were considered as two separate phases. The mechanical properties of the crystalline phase were modeled using viscoplastic constitutive equations, which take dislocation density evolution and diffusion creep into account, while an elasto-viscoplastic model based on diffusion mechanism was used for the grain boundary phase. For microcrystalline materials, the surface-to-volume ratio of the grain boundaries is low enough to ignore its contribution to the plastic deformation. Therefore, the grain boundary phase was not considered in microcrystalline materials and the mechanical properties of the crystalline phase were modeled using an appropriate dislocation density based constitutive equation. Finally, the constitutive equations for polycrystalline materials were implemented into a finite-element code and the results obtained from the proposed constitutive equations were compared with the experimental data for polycrystalline copper and good agreement was observed. Dislocation density based constitutive modeling had a good capability and reliability for predicting the mechanical behavior of coarse-grained or fine-grained metallic materials, so these equations were used for dual phase steels. While the grain boundary phase was separated of ferrite and martensite phases, the geometry of the dual phase steels microstructure was modeled with voronoi method. The Gurson-Tevergaard-Needleman damage model, the dislocation density based model and elastic-perfect plastic model were used for grain boundary, ferrite and martensite phases respectively and the mechanical behavior of dual phase steels were investigated by finite element method in uniaxial tension test. The obtained results show that the grain level inhomogeneity plays a main rule in plastic deformation of these materials. Finally, crystal plasticity constitutive equations were used to investigate the crystalline direction effect and material texture. Voronoi method was used for simulating the non-homogeneity of the microstructure in plastic deformation. In addition, the elastic modulus parameters for the model were obtained by molecular dynamic simulations. The plastic deformation of Fe metal was simulated with the finite element method and good agreement was observed with the available experimental data. Keywords : Polycrystalline material, Dislocation density based moadel, Dual phase steel, Voronoi method, crystalline plasticity