An iterative transmission line technique for the determination of complex permittivity of non-magnetic, isotropic materials is addressed. The method is based on minimizing the objective function measuring the error between simulated and measured scattering parameters by classical Newton's method. An Electromagnetic field solver is used to solve the forward problem which is repeated in the application of Newton's method at each iteration. Since full-wave simulation is carried out, the method has no strict limitations on the geometry of the samples. For validation, proposed method is tested against partially loaded waveguide measurements at K-band.