Surface refractive index of a monotonously graded-index waveguide, which is greater than the effective index of the fundamental mode and cannot be measured in conventional m-line spectroscopy, is experimentally determined by employing the surface plasma wave (SPW) resonance technique based on analytical transfer matrix (ATM) method. The index profiles predicted are more accurate than those obtained by the conventional inverse WKB (Wentzel, Kramers, Brillouin) method.