For the design of the multilayer ceramic capacitors (MLCs) with core-shell structure, the solubility modes of La-Mg and La-Al into BaTiO3 were clarified as a function of composition based on the lattice parameters' change. The lattice parameter was determined based on a cubic system at a higher temperature than the Curie temperature in order to avoid the crystal structure change. On the BaTiO3-La2O3-MgO system, there were three substitution modes. In the region where x was small on the (Ba1-xLax)(Ti1-x/2Mgx/2)O-3, Mg ions preferentially dissolved into B site of BaTiO3, but in the region where x was large, both La and Mg ions dissolved into A and B site, respectively. On the other hand, on the BaTiO3-La2O3-Al2O3 system, La and Al ions partially replaced Ba and Ti ions, respectively, according to pre-determined replacement formula (Ba1-xLax)(Ti1-xAlx)O-3. The solubility range of La into BaTiO3 was wider than that of Dy or Ho which make the core-shell structure. So, the dielectric materials composed of BaTiO3 containing La are not fitted to form the core-shell structure. Therefore, considering ionic radius of rare earth element, BaTiO3 containing rare earth elements whose ionic radii are much smaller than that of Ba is fitted to form the core-shell structure.