The micro-mechanism of precious metals trapped in pyro-metallurgical processes by base metals or their mattes were discussed in present paper .It was forwarded that the reasons for trace amount of precious metals trapped into base metals or matteswere that the melted slag phases possessing very different structure and properties from that the melted base metal or matte phases.For the slag phase, its formation is thought depending on the covalent bonds and the ionic bonds between Si, O atoms and Ca²⁺, Mg²⁺, Fe²⁺ ions to tie up between each other .Because the bonding electrons in the slag phases are all thought as the localized electrons, the precious metal atoms are not stable among them .However, for the metal phase, its formation is thought depending on the metal bonds to bind up together the metal atoms.And because the electrons in the metal phases can move freely among the atoms, the precious metal atoms entering into the metal phase can cause the system free energy decreased .For the melted mattes phase, because it has high conductivities (between 103~104 S·cm -1) at the higher temperature, and its temperature coefficients present the negative values, its conductance belongs to the electron conductance .The properties of the matte are in fact similar with that of metal at their melted state, therefore precious metal atomswill enter into the matte phase, but not the slag phase in matte-smelting process .