A plasma cathode ion source has been developed to attain a long lifetime for oxygen ion production. In this ion source, an Ar plasma which is non-reactive plasma serves as a cathode for a thermionic hot cathode used in the Kaufman ion source. This ion source consists of two compartments: (1) a plasma generator, and (2) a plasma chamber. The plasma generator is used for generating a plasma cathode, while the plasma chamber is used for producing oxygen ions. Two chambers are connected to each other by an anode hole which has narrow tapered ducts. These two chambers are differentially pumped out, the pressure of the plasma generator has to keep a high pressure, so that the large pressure difference between two chambers prevents oxygen gas from flow into the plasma generator. Oxygen gas fed to the plasma chamber does not damage the thermionic hot cathode located in the plasma generator. It has been confirmed that the plasma cathode ion source can produce a stable ion beam and has a long lifetime. Mass spectrometry results show that this ion source has an ability of generating a considerable amount of positive oxygen ions.