A theoretical analysis of a resonant-cap IMPATT oscillator has been performed on the basis of a device-circuit interaction where the resonant-cap is considered as a radial-line cavity around the diode. the theoretically analysed results have been evaluated through computer-aided solutions. It is found that to obtain the optimum performance of the oscillator with a given device there are optimum cap-circuit dimensions, i.e. a (D-h) combination for which the load impedance terminating the resonant-cap cavity nearly equals the characteristic impedance of the rectangular waveguide in which the resonant-cap is mounted. Also the theoretical computations show that the ratio of the physical cap diameter (D) and cap height (h) to the wavelength of oscillation (λr) is nearly 0.36 and 0.08, respectively, while the effective electrical cap radius (r eff) is approximately a eff quarter-wavelength. These results are in good agreement with the previously reported experimentally measured values. the detailed theoretical analysis and the corresponding computed results reported in this paper are expected to be a good basis for the design and understanding of the properties of the cap-type microwave and millimetre-wave sources. © 1997 Taylor and Francis Group, LLC.