The phase evolution, nucleation, and sintered ceramics of barium titanate (BaTiO3, BT) powder prepared by solid-state synthesis with an ultrafine starting material (27 m2/g of BaCO3 and 190 m2/g of TiO2) were investigated in this study. Surface diffusion between BaCO3 and TiO2 was observed at a relatively low temperature of 400 °C by transmission electron microscopy. Rapid nucleation of the BT and cubic BT phases was observed at 500 °C by x-ray diffraction. The derivative thermogravimetry curve clearly shows a single step of BT formation at 600 °C. In short, pure BT particles with an average particle size of 250 nm and high tetragonality were prepared by solid-state synthesis, which produced X7R ceramics with high dielectric permittivity, high insulation resistance, and a clear core–shell structure.