An optimization study on crystallization annealing of (Ba,Sr)TiO3 (BST) thin films, grown by metalorganic chemical vapor deposition at a wafer temperature of 420 °C, was performed. The as-grown film had an amorphous structure with a dielectric constant of about 20. The annealing parameters, including temperature, atmosphere, time, and sequence, were varied considering the limitations imposed by integration processes for ultralarge-scale integrated dynamic random access memory devices. The dielectric constants of the crystallized films were largely determined by the maximum temperature that the films experienced with minor effects from the annealing atmosphere. However, the leakage current densities were quite dependent on the annealing sequence and atmosphere. It was concluded that the major factors which determine leakage characteristics were concentrations of impurities, especially carbon, oxygen vacancies, and interfacial defects caused by top electrode sputtering.