The effect of the surface roughness on the characteristics of self-assembled monolayers (SAMs) was quantitatively examined by measuring the photocurrent efficiency at a gold electrode modified with a SAM of a porphyrin-ferrocene-thiol linked molecule as a function of the surface roughness. The photocurrent efficiency was higher for the flatter substrate. This result confirms the importance of the substrate's surface roughness to achieve the ordered arrangement of the functional moieties of the SAM, i.e., effective control of the electron/energy transfer.