In terms of the mode eigenvalue equations and field distributions, calculations are performed for Au/InGaAsP/InP, air/Au/InGaAsP/InP, and Au/SiO2/InGaAsP/InP periodic optical waveguides. On the basis of the computed results, the coupling property in the periodic structure is analyzed, the mode split and the mode band formation of both TE and TM guided modes are described, and the effects of the thicknesses of the metal cladding and the oxide buffer layer on mode propagation and absorption loss are discussed for TE and TM guided modes as well as for the TM surface-plasma mode. Furthermore, the conversion from the surface-plasma wave to the guided wave for the TM mode is explained for Au/SiO2/InGaAsP/InP periodic structures.