Abstract: Due to the well-known effect of large specific surface area of nano-materials, they tend to show distinct properties other than bulk materials at large scale, especially the nanoparticles of metals which have been paid much attention. Most of metals consist of face-centered cubic （one of close packing lattices） unit cells. In this pa- per, an approach to calculate the ratio of numbers of surface atoms to interior atoms in a bulk materials with face- centered cubic lattice is proposed. Supposing the metallic particles consists of repeated unit cells that are stacked e- qually in the three crystal axes directions, a recursion formula can be addressed on how to calculate the numbers of surface atoms to interior atoms of particles with different sizes. According to the method, the size of a certain material that shows considerable effect of specific surface area can be estimated.

Key words: nanoparticles of metals, face-centered cubic lattice, specific surface area