This study proposes a model to measure the real and imaginary parts of the permittivity of a dielectric material using the S -parameters of a coplanar waveguide (CPW), and discusses the detailed derivation and application of the proposed model. Based on this model, the permittivity of the substrate material is calculated at 200 GHz using the measured S-parameters of the CPW. The calculated result agrees well with the theoretical value. The proposed model can be employed to characterize the dielectric properties of many materials within the sub-terahertz frequency regime.