Abstract: This paper proposes a rectangular spiral microstrip resonator with a thin - film resistor loading and its application to microstrip equalizer. A miniaturized microstrip planar equalizer substructure incorporating this thin-film resistor loaded spiral resonator is designed and simulated and the loading position is also studied. By loading a thin-film resistor in the spiral resonator, an adjustable quality factor of the resonator is obtained. A High Frequency Structure Simulator is used to show that a microstrip equalizer incorporating this thin-film resistor loaded spiral resonator not only can be frequency adjustable, but also can be attenuation adjustable and quality factor adjustable. Simulation results show that this thin-film resistor loaded spiral resonator is suitable to microstrip equalizer.