We fabricated Josephson junctions having an epitaxial NbN/MgO/NbN microstrip resonator to evaluate the RF performance of the epitaxial NbN/MgO/NbN trilayers. All of the NbN and the MgO layers that formed the junctions and the resonators were confirmed to be epitaxial growth by X-ray analysis. The width of the microstrip resonator was 0.7 μm and the length was changed from 1000 to 99.3 μm. At the resonant frequency, current steps appeared in the I– V characteristics. The steps were observed up to 2.5 mV. By fitting the simulated results to the measured data, the surface resistances of the epitaxial NbN thin films were estimated to be about 35–88 m Ω in the 0.7–1.1 THz range.