Dielectrics composed of BaTiO3-rare earth oxide-MgO have been studied to design a non-reducible low-loss dielectric material. Curie temperatures of dielectrics composed of BaTiO3–Gd2O3–MgO were lowered by Gd2O3, which provided the dielectrics with low power loss. It was found that the addition of BaZrO3 to BaTiO3–Gd2O3–MgO decreased the capacitance change with temperature. Gd2O3 diffused easily into BaTiO3, but, it could not easily diffuse into BaTiO3 to which BaZrO3 had been added. It was considered that BaZrO3 prevented the Gd2O3 diffusion into BaTiO3, resulting the remains of ferroelectricity and reduced capacitance change. Thus non-reducible and low power loss X7R dielectric ceramics were developed.