A technique for the design of circular- and racetrack-shaped NRD guide ring resonators was developed based on the mode coupling theory. Besides the operating mode, a parasitic mode is generated at curved sections of the resonator as a result of the mode conversion. Resonance of the NRD guide ring resonator is derived by characteristic equations of the coupled modes and then employed in making the design diagrams, which are useful for determining the dimensions of the ring resonators. It is shown that the discrepancy between the experimental results and previous theory can be resolved by using the present theory. Low loss, small-sized ring resonators with curvature radii of less than 5.3 mm were fabricated at 60 GHz and a band rejection performance of more than 30 dB was observed. Moreover, a procedure for the design of the channel dropping filter was developed and a 2-pole filter, which has great advantages such as a low insertion loss of 1.2 dB and a compact size smaller than that of a golf ball, was successfully developed by using two racetrack-shaped ring resonators.