Structures of grain boundaries of SrTiO3 semiconductive capacitor have not been fully elucidated. Several studies were focused on the SrTiO3 systems which include additives, such as, Al2O3 or SiO2. In this study, we first investigated the grain boundaries of Sr0.975Ca0.025TiNb0.004O3 which did not contain any additives. Then Bi2O3/CuO, which is a grain boundary modifier, was also added in order to investigate the process that changes grain boundaries into insulators. Samples heat-treated with Bi2O3/CuO were quenched in water and TEM (transmission electron microscopy) was carried out on these microstructures. Grain boundaries of samples sintered in a reduced atmosphere showed segregation of TiO2 oxide phase, which included no traces of Sr. On the other hand, in grain boundaries of samples heat-treated with the Bi2O3/CuO agent, at least two phases existed. One phase included large amounts of Bi and Cu, while the other phase showed only traces of these elements. Near the grain boundaries Bi and Cu diffused into the matrix grains showing inhomogeneous diffusion of which diffusion lengths were different at each side of the grain boundary and seemingly lattice coincident with both grain boundary layers and matrix grains. In addition, steps of about 2nm in height were observed at grain boundaries.