Knowledge of materials behaviour placed in an electromagnetic field was found to be of immense significant especially as it is attributed to military hardware, electronics, communication and industrial applications. An overlay microstrip antenna was employed in this study to determine scattering parameters of an overlay microstrip antenna using finite element method (FEM). The substrate thickness used on the microstrip was 10 mm thick and 8 x 12 cm (width and length). All simulation was carried at X band frequency range (8-12 GHz) using FEM. The result amongst others showed that the transmission coefficient depends on the permittivity of the sample. The result also showed that lowest permittivity of material produced the greatest transmission coefficient. It is then concluded that the results obtained for the transmission and reflection coefficients can be used for the prediction of complex permittivity of materials used as overlay on a microstrip antenna.