Basing on ab initio methods and the elastic scattering Green function theory, we have investigated electronic transport properties of metal_molecule_metal junctions by taking the 4_4′_dimercaptodibenzene molecule as an example. The molecular geometric structures, electronic structures, and current_voltage characteristics have been studied for varying the distance between the two electrodes. Numerical results show that the changes of the distance between electrodes give various influence on the extended molecule's geometric and electronic structures, which brings effects on the electronic transport properties of the molecular systems. The equilibrium state of the extended molecule is not the best situation for electronic transportation. The characteristics of electronic transportation can thus be improved by adjusting the distance between the two electrodes.