The electron transport via a series of porphyrin molecular wires coordinated with various metal cations (Zn2+, pt(2+), Pd2+, and Ni2+) has been investigated with the metal-molecule-metal junctions by using the first principle approach, on basis of density functional theory and nonequilibrium Green's function calculation. The metal cation effect on the transportation behavior has been compared and discussed in terms of the electronic structures and transmission spectra. The results demonstrate that the coordination of metal cations may decrease the molecular conductance.