One of the characteristic features of the metil claded dielectric waveguides different from the simple dielectric waveguides is that the former can excite guided waves with effective mode index N<1, which has the propagation constant less than the wave-number of the light in vacuum: ?/c. Authors measured directly the dispersion properties for N>1 and N<1 modes in the metal claded dielectric waveguides by the frequency scanning ATR method. The experimental results are in agreement with the theoretical calculations. It is found that the coupling efficiency through the prism coupler to the metal claded dieleotrio waveguide is depending on the thickness of iihe metal coupling layer.The coupling efficiency reaches to the maximum when the thickness of the coupling layer is about 13.0 nm. The iihickness of the coupling layer will influence the mode index of the waveguide owing to the slight changes of the dieleotrio constant of the coupling layer versus its thickness during their evaporation deposition in a vacuum coating system.