A system and method for identification of complex permittivity associated with a transmission line dielectric is proposed. A network analyzer measures scattering parameters over a specified frequency band for at least two line segments of different length and substantially identical cross-section filled with a dielectric. A first engine determines non-reflective (generalized) modal scattering parameters of the difference segment based on the measured scattering parameters of two line segments. A second engine computes generalized modal scattering parameters of the line difference segment by solving Maxwell's equations for geometry of the line cross-section with a given frequency-dependent complex permittivity dielectric model. A third engine performs optimization by changing dielectric model parameters and model type until the computed and measured generalized modal scattering parameters match. The model that produces generalized modal S-parameters closest to the measured is the final dielectric model.