In the work, measured results of complex permittivity of some commonly used building materials have been presented. The relative permittivity was measured in the coaxial line with TEM fundamental mode. Such a measurement condition corresponds to the free-space test, with a planar incident wave. In the literature different techniques were presented for coaxial lines permittivity determination, basing on measurements of complex values of scattering parameters. In proposed attempt the only magnitude of reflection and transmission coefficients have been used for relative permittivity determination. The relationship for calculation of the relative permittivity and analysis concerning the algorithm convergence has been discussed. Proposed calculation method allows applying this method for measuring the electric field, neglecting the phase assessing. As an example of material measurement the relative of permittivity of two kinds of bricks, concrete and cellular concrete were presented.