The effect of CO adsorption on the electron transport behavior of single Fe-porphyrin molecular wire with sulfur end groups bonded to two gold (1 1 1) electrodes is investigated using nonequilibrium Green's function formalism combined with first-principles density functional theory. The currentvoltage characteristics of the single Fe-porphyrin molecular wires with and without CO adsorption are calculated. The results demonstrate that Fe-porphyrin molecular wire shows a negative differential resistance (NDR) at 2.0 V. The molecular current through Fe-porphyrin is significantly reduced after CO adsorption. Such a significant difference indicates the potential application of Fe-porphyrin as a molecular sensor and/or a molecular switch. The molecular projected self-consistent Hamiltonian (MPSH) states and transmission coefficients of the single Fe-porphyrin molecular wires with and without CO adsorption are analyzed. It is found that the changes of the MPSH states of the single Fe-porphyrin molecular wires with and without CO adsorption lead to the switching behavior. Furthermore, the transmission coefficients of the single Fe-porphyrin molecular wires with and without CO adsorption under various external voltages are also investigated. The results show that the transmissions through the highest occupied MPSH and the lowest unoccupied MPSH states of the single Fe-porphyrin molecular wire are suppressed significantly at this external voltage of 2.0 V, which causes the NDR.