A silicon-clad planar diffused optical waveguide structure is proposed and propagation characteristics for a wavelength of 0.6328 μm are investigated. The refractive index profile of the substrate is assumed to be a Gaussian profile and a maximum index change of 0.01 is also assumed. Computer modelling studies indicate that silicon-clad diffused optical waveguides exhibit a damped periodic oscillation in the attenuation and mode index curves. The attenuations of these waveguide structures are relatively small compared with those of the four-layer planar homogeneous waveguide structures reported by Batchman and McWright. The attenuation may be reduced with a low-index buffer layer. Furthermore, the attenuation and mode index are significantly altered by the conductivity changes of silicon.