AbstractIn the superconducting cavity stabilized oscillator (SCO), which has extremely high‐frequency stability, the power input to the superconducting cavity (SC) must be minimized since the energy stored in the SC causes a shift in the resonance frequency away from its value at zero stored energy due to the electromagnetic radiation pressure and nonlinear surface reactance. Therefore, in the SCO with self‐injection locking, a loosely coupled directional coupler is used to couple a free running oscillator to an SC. This paper describes the design of self‐injection‐locked SCO's.The effects on frequency stability of the SC coupling factor and electrical line length between the SC and oscillating device are discussed. Oscillation frequency is given as a function of electrical line length. Optimum line length is discussed and the stability criteria for the oscillating device are derived. Based on this design consideration, an X‐band Gunn effect oscillator is stabilized. The SC is fabricated from solid Nb and has an unloaded Q‐factor of 6.7 × 106. The noise floor of frequency stability as low as 8.5 × 10‐11 has been obtained.