The transport property of silicon sandwiched between Au (100) and Au (100) is investigated with a combination of density functional theory and non-equilibrium Green's function method. It is found that the conductance decreases with distance increasing. When dz =9.72 Å, the structure of junctions is the most stable and the conductance is 1.227G0 (G0=2e2/h) , which is contributed by the px, py and pz electron orbits of silicon atom. The I-V curve of junctions in stable station show linear characteristics under external bias vottage. With the increase of an external positive and negative voltage, the conductance decreases slightly, and the asymmetry change appears.