A new approach to modeling of the plasticity of materials with nanostructure and ultrafine one has been proposed. Its main advantage is the minimum number of physical parameters in use. In the context of the proposed model, we calculated the volumetric density of the energy of surface tension of the material grains. This energy is a significant part of the internal energy during deformation. The size dependence of the melting temperature of nanoparticles was compared with experimental data. We obtained size dependence of the yield point on its basis. Yield point was interpreted as the result of changes of grains surface energy during the deformation. The obtained yield point dependence on the grain size was related to the Hall-Petch law, and this resulted in confirmation of the hypothesis on the crucial role of surface tension forces in the initial stage of plastic deformation of ultrafine materials.