This study investigates the effects of doping BaTiO3 with MgO and Y2O3 on the formation of core-shell structure. The MgO and Y2O3 enhanced the shrinkage upon sintering and inhibited the grain growth, respectively. However, increasing the amount of Y2O3 to 3.0mol% suppressed the shrinkage upon sintering. The results of the diffusion experiment revealed that Y3+ was dissolved in the BaTiO3 lattice to a depth of 5-10nm inside the grains, whereas Mg2+ tended to remain close to the surfaces of the grains when sintered at 1150 degrees C for 18h, suggesting that Y3+ may have had a higher diffusion rate than Mg2+. The Mg2+ prevented the diffusion of Y3+ into the core during sintering. Therefore, Mg2+ plays an important role as a shell maker in the formation of the core-shell structure in the codoped system. The core-shell structure can be obtained in BaTiO3 ceramics that are codoped with MgO and Y2O3 upon sintering at 1150 degrees C for 3h.