The electrical transmission of a series of oligo(p-phenylene) dithiolate (PDT) wires coupled with gold electrodes is investigated using a Green function method combined with a hybrid density functional method. To investigate how the nonplanarity of the pi-backbone affects the electrical transmission, two types of PDT wires with planar and nonplanar forms are compared. The exponential decay of conductance at the Fermi level G(E-F) = G(0) exp(-gammaL), is observed for both forms. The damping factors, gamma's, for the planar and nonplanar PDT wires are 0.165 and 0.256 Angstrom(-1), respectively, these values being comparable with experimental data for analogous oligophenylene wires. A possible reason for the slower decay in the planar PDT wires is analyzed in terms of molecular orbitals.