This paper presents an analytical method for the extraction of the complex relative permittivity of multi-layered dielectric substrates that are used to construct thin film microstrip lines (TFMLs). The proposed approach is based on the analysis of an equivalent parallel plate waveguide filled with a dielectric material whose relative dielectric permittivity is the effective dielectric permittivity of the corresponding TFML. The extraction technique relies only on the knowledge of the complex propagation constant obtained from S-parameter measurements and is applicable to a wide range of line dimensionsand dielectric permittivity values. For experimental verification, several TFMLs were fabricated, and from the measured scattering parameters, the complex dielectric characteristics of each device are extracted.The extracted relative permittivity and loss tangent values are then compared to the dielectric parameters obtained from a numerical technique based on full wave analyses. The accuracy of the proposed analytical method is shown to be approximately 15% and 5% for the fabricated TFMLs with ratios of the microstripline width (W) to dielectric thickness (H), (W/H) of 4 and 80, respectively.