Abstract: An equivalent transmission line and network with two ports constructed in the investigation of the electric circuit has been successfully applied to the problems of constitutive parameter retrieval for materials in electromagnetic fields and waves,and based on transmission-line and network theory,the relations between the scattering matrix and relative constitutive parameters of material are derived.The circuit and electromagnetic simulation results presented show that the relation among relative permittivity,relative permeability and reflection coefficients,transmission coefficients,sample thickness is correct.