The prism coupler method has been applied to the investigation of refractive index profile (RIP) in liquid crystal (LC) layer. Waveguide modes have been excited in thin LC layer arranged as a planar wavequide. Modal lines of the modes excited in the waveguide are observed as evanescent in dielectric prism by tunnelling effect. Measured position of each modal line provides a mode propagation constant value. In a multimodal waveguide the set of the propagation constants creates data for RIP determination by means of M-line procedure. The usefulness of the M - line method to the investigations of LC waveguides is illustrated and verified by nematic liquid crystal (NLC) elastic response measurements.