We have developed a new method to investigate the dielectric and interfacial properties of gate dielectric thin films by microwave measurement. BST thin films were deposited on 10 Ω-cm (normal) and 10 kΩ-cm (high-resistivity, HR) silicon substrates at the same time by RF magnetron sputtering. For the BST/HR-silicon, coplanar waveguides (CPW) were fabricated and measured at microwave frequencies with Thru-Reflect-Line (TRL) calibration while CV measurements were carried out for BST/normal-silicon. From the phase change of CPW transmission line and the maximum capacitance in CV measurement, the dielectric constants of both the BST thin film and interface layer can be determined. Furthermore, the behaviors of insertion loss versus bias voltage were found to be correlated with the trap states density. The results indicate that our method can provide useful information to study the dielectric and interfacial properties of metal-insulator-semiconductor (MIS) structures.