The research on alloy materials used in vacuum circuit breakers with silver and palladium was to analyze and compare the instability of a contact surface between silver and palladium as an alloy cathode in vacuum, the different ratio of silver and Palladium were performed at 1%, 2%, and 4% by using the cathode spot model. The transient recovery voltage was predicted. It was found that the transient voltage is decreased with increasing the percentage of an amount of palladium in the cathode. The cathode spot region is recognized as the collision less space charge sheath connected with singly ionized collisional plasma.