Continuous coherent microwave oscillation is observed in InSb p-n junctions under the application of a dc reverse bias voltage in the region of avalanche breakdown and an external high magnetic field at 77 K. The observed oscillation frequency varies with the sample current of the p-n junction and the magnetic field. The oscillation mechanism seems to be different from that of impact-avalanche transit-time (IMPATT) diodes because the external high magnetic field is required to cause the microwave oscillation in the InSb p-n junctions.