The specimen current stability of single‐crystal and of sintered LaB6cathodes are compared from the viewpoint of their dependence on the cathode tip temperature and the vacuum pressure. The specimen current of the single‐crystal LaB6cathode is very stable during degradation of the vacuum in the gun chamber, while in the sintered LaB6cathode it becomes very unstable in poor vacuum even though the total emission current is stable. The cause of this remarkable difference between the single‐crystal and sintered LaB6cathodes is discussed.