Based on the relationship between the volume of metal valence electrons occupying k space and thevolume of the Brillouin zone , the radius of the Fermi sphere was calculated, and the formula for calculatingelectron mean free path was derived, Taking Ag and Cu as examples, the electron mean free path wascalculated, which was consistent with the results obtained by other methods.When the geometric scale of metalmaterials was equivalent to the electron mean firee path , the physical properties related to it would undergo significant changes, From this perspective ,the critical values of Ag and Cu nano size were estimated using theaverage free path seale of electrons to be 52 nm and 39 nm , respectively.This caleulation approach was alsoapplicable to other metal nanomaterials.