When barium titanate was synthesized in soild-solid reaction the abnormal expansion occurred from 900$^{circ}C$ to 1100$^{circ}C$. The equi-molecular mixture of BaCO3 and TiO2 was sintered from 800$^{circ}C$ to 1300$^{circ}C$ on the condition of air, vacuum and CO2 atmosphere. After that the specimens were tested closely with XRD, Dilatometer, SEM and EDS. The result indicated that; 1. The crystal phase which was concerned with expansion of BaTiO3 was Ba2TiO4 as the intermediate crystal phase. 2. The formation of Ba2TiO4 was affected by the firing atmosphere. 3. The expansion occurred when BaTiO3 changed to Ba2TiO4 and pore also expanded by the expansion of BaTiO3 body just as the model of expansion.