The reflection and transmission coefficients (S11 and S21) of planar material samples are determined using the proposed model. The complex permittivity and complex permeability are calculated from the values of S11 and S21. The proposed model is tested with different materials in the frequency range of – GHz. The results show that the more dielectric loss tangent, the more accuracy of complex permittivity. However, the complex permeability is slightly effected by the magnetic loss tangent. | Modeling method for determining complex permittivity and complex permeability of materials using electromagnetic wave propagation in free space at X-band