The constituting atoms of material interact with each other and prevent the rupture of the material. Meanwhile, the bulk atoms are more constrained than the surface atoms because the bulk atoms, relative to the surface atoms, have more atoms in their neighbors. Because of this fact, creating a new surface, needs to consume energy. This energy that can be calculated is called the surface energy. Surface energy is a basic property for different crystal plane that helps to understand various phenomena such as surface separation, roughness, catalytic activity and stability of crystal structure. In fact, when thermodynamics or any other feature of nanostructures is discussed, the surface energy is one of the key properties. This property at nano-scale has the utmost importance, since at this scale the surface to volume ratio is very large and thus there is a significant difference between nanoscale properties and the engineering scale properties. In this study, the surface energy of metallic nanoplate of gold, silver and iron using molecular dynamics and continuum mechanics models are investigated and shown to be dependent on size. Metallic nanoplate surface energies of various materials with different thicknesses were determined using molecular dynamics and it was shown that for Metallic nanoplate with sufficiently small thickness, the surface energy is dependent on the thickness of nanoplate and the surface energy is reduced by reducing the thickness of the nanoplate. By analyzing the excess energy of different layers in nanoplate it was found that this size dependent is due to the excess energy reduction in the surface layers and rising in the inner layers that totally reduces the surface energy. Nano-scale molecular dynamics simulation is time consuming. The analysis of this phenomena is very important from a theoretical point of view. Therefore, the surface energy using continuum mechanics model was also analyzed. Size-dependent surface energy and excess energy density distribution curves of gold, silver and iron nanoplates are investigated using continuum model analytical relations and it was shown that the results have a good agreement with molecular dynamics simulation. The results indicate that the accuracy of the continuum analytical model for size-dependent surface energy is satisfactory. Keywords: Size-Dependent Surface Energy, Metallic Nanoplates, Molecular Dynamics Simulations, Continuum Mechanics