In order to clarify the decrease of the dielectric constant of Ba0.5Sr0.5TiO3capacitor under the electrical stress, the degradation phenomena including resistant degradation were studied in detail. They were well explained by the increases of the dielectric relaxation and dielectric dispersion which are caused by the generation of oxygen vacancies. This explanation was proved by several additional experiments such as the investigation of Schottky barrier with the stress, the observation of some bubbles and O2plasma annealing. It was also confirmed that the electrical degradation of BST capacitor is effectively suppressed by using iridium (Ir) electrode, especially as a top electrode.