Using a high‐temperature superconducting YBa2Cu3O7−&dgr;Josephson‐junction harmonic mixer operated at 77 K and an analog phase‐locking loop, we have locked a 3 mm waveband Gunn oscillator to a synthesizer for harmonic numbers ranging from 10 to 52. The spectrum of the phase‐locked intermediate frequency reveals good phase‐locked quality even for the 48th harmonic mixing between the signal at 104.3 GHz and the local oscillator at about 2 GHz. Our experimental results show that a highTcsuperconducting harmonic mixer can work efficiently with high harmonic number for phase locking in the millimeter waveband. ©1995 American Institute of Physics.