The sintering characteristics and the reaction of additives with BaTiO3 (BT) were examined for two materials having Nb-rich composition (Comp.N) and Go-rich composition (Comp.C) to elucidate the relation between the stability of the core-shell microstructure and the Nb/Co ratio in the BT-Nb2O5-Co3O4 system. TEM observation revealed that the concentration gradient of Nb and Co existed in the shell region although Nh and Co macroscopically distributed homogeneously. X-ray diffraction analysis showed that the shell formation preceded the densification and completed at about 1280 degrees C for both Comp.N and Comp.C as determined from differential scanning calorimetry. A diffusion couple experiment disclosed that Co had a larger diffusivity than Nh and that the diffusion of Co was suppressed when the sample was codoped with a sufficient amount of Nb. On the basis of these experimental results, new mechanisms of the formation and collapse of core-shell structure in the BT-Nb2O5-Co3O4 system were proposed.