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The formation process of BaTiO_3 from powder compacts, equi-molar mixture of BaCO_3 and TiO_2, and an abnormal expansion during the formation process were investigated. Powder compacts were fired for 30 min in air atmosphere at various temperatures, up to 1200℃. The formation process of BaTiO_3 is devided into three steps; firstly, BaTiO_3 is formed at the contact points between BaCO_3 and TiO_2 particles, secondly, Ba_2TiO_4 is formed by the reaction of the prior-formed BaTiO_3 with BaCO_3, finally, BaTiO_3 is formed by the reaction of residual TiO_2 and Ba_2TiO_4. In the macroscopic observation, it was confirmed that a powder compact of BaCO_3 on magnesia and rutile substrates begins to melt at 950℃. This liquid phase is thought to influence the formation reaction of BaTiO_3 greatly. It is well known that Ba component diffuses to TiO_2 one-sidedly in this reaction. As a result, the space where BaCO_3 occupied in a compact specimen remains as a pore after the termination of reactions. Then, an increase in the volume happens by TiO_2's changing in BaTiO_3. These are considered to be the cause of an abnormal expansion caused by the reaction of BaCO_3 and TiO_2.