The electrical and dielectric properties of Ba0.3Sr0.7TiO3 (BST), as a function of annealed temperature and oxygen partial pressure (P(O2)) , were studied in this paper. The resistivity data were fit to a p type model involving only doubly ionized oxygen vacancies, holes, and accidental acceptor impurities, and it decreased as P(O2) and temperature increased. The breakdown strength exhibited the same trend with resistivity. The breakdown strength exhibited the same trend with resistivity. The maximum dielectric constant of BST was observed under the lowest P(O2) at the temperature of 900°C. BST annealed under P(O2) of 0.1 atm at the temperature of 800°C obtained the highest volume energy storage density.