The effect of magnetic field on the reverse characteristics of silicon alloy junction diodes in the breakdown condition has been studied. The breakdown voltage caused by internal field emission has been found to remain unchanged under a magnetic field (maximum field used 15 kOe). On the other hand, the breakdown voltage occuring by avalanche ionization increased by about 15 mV on the application of a magnetic field. In the post‐breakdown region, while the magnetic field caused no change in current in diodes breaking down by the former mechanism, it increased the current (at constant voltage) in diodes having avalanche type of breakdown. Explanations of the observed results have been given.