In surface resistance measurement of high Tc superconducting (HTS) thin films, the conventional resonant perturbation methods have large uncertainties, because their calibrators usually have much larger surface resistance than HTS thin films. This article describes a new type of calibrator, mirror-image calibrator: when the open end of a dielectric resonator is connected to its mirror image, the resonant frequency and quality factor of the resonator are equal to those of the dielectric resonator when its open end is shorted by a perfect conductor. This principle is applied to modify the dielectric resonator method in surface resistance measurement of HTS thin films. The structures of the dielectric resonator for surface resistance measurement (Rs probe) and its mirror-image calibrator are explained in detail. Comparison is made between the present technique and the conventional perturbation technique, and results show that the accuracy and sensitivity of the resonant perturbation method are greatly improved by using the mirror-image calibrator. © 1999 American Institute of Physics.