The effect of molecular orientation on the electron transport behavior of single porphyrin sandwiched between two gold (111) electrodes is investigated by density functional theory calculations combined with non-equilibrium Green's function method. The results show that the porphyrin with parallel connection to gold (111) electrodes is more conductive than the porphyrin with diagonal connection to gold (111) electrodes. The mechanism of the difference of electron transport for these two molecular junctions is analyzed from the transmission spectra and the molecular projected self- consistent Hamiltonian states. It is found that the intrinsic nature of the molecule, such as the pi-conjugated framework and the strength of molecule-electrode coupling, are the essential reason for generating this difference of electron transport for the two molecular systems. (C) 2011 Elsevier B.V. All rights reserved.