Abstract: In this paper, the effect of the depth of SiO₂ layer on the effetive refractive index and the damping rate of electrical field amplitude of guided modes in Si-SiO₂-guiding layer-air cladding layer structure is discussed. It makes use of the method dealing with the effect of coupling prism on guided modes of the planar waveguide in strong coupling condition. The numerical solution is given for a concrete situation. The phenomena were observed and described experimentally that the damping characteristic lengths of various modes in a waveguide are different.