When the barium titanate is Synthesized in Solid-solid reaction the abnormal expansion occurs at about 110$0^{circ}C$. The mixture was made of corresponding to the theoretical composition of barium titanate. The mixture was fired at various temperature from 90$0^{circ}C$ to 130$0^{circ}C$, After that the specimen was tested closely with XDR dilatometer and SEM. The results indicate that 1, The activation energy of barium titanate formation was 42 Kcal/mole. 2, Thermal expansion up to to 90$0^{circ}C$ was mainly caused by $CO_2$ whereas it was mainly caused by the formation of the secondary phase above 100$0^{circ}C$. 3. Thermal expansion was not influenced by the forming pressure up to 1000kg/$cm^2$ but it was largely influenced bythe forming pressure above 200kg/$cm^2$