Abstract: Based on the first principle，electrical properties of a molecular junction consisting of pyrene-1，8-dithiol molecule and gold surface have been investigated. The cluster of three gold atoms is used to simulate the gold surface. Density functional theory is employed to obtain the electronic structures of the molecule and the extended molecule. Then the frontier orbital theory and the perturbation theory are used to determine the interaction energy between the molecule and the gold surface quantitatively. The elastic Green function method is applied to study the current-voltage properties of the molecular junction. Numerical results show that the sulfur atoms can be chemically absorbed on the gold surface and the bonding between the molecule and gold is mainly covalent-typed. The fermienergy of the extended molecular system lies between the HOMO and the LUMO and closer to the HOMO of the system. When the external applied bias is lower than 1 V，there is a current gap for the molecular junction. With the increasing of the bias，the conductance of the junction exhibits plateaus. These electrical properties are closely related with the electronic structures of the molecular junction. The extended molecular orbits have great contribution to the charge transport. Localized molecular orbits give little contribution to the current while charge transport is taken place by tunneling.