We have developed an apparatus, which can measure heat flux and dielectric constant simultaneously. The heat flux and the dielectric constant of a single crystalline BaTiO3 were simultaneously measured with a temperature resolution of milli-Kelvin to study the detailed behavior of the phase transition. The anomalies are observed between 400 and 402 K both in the heat flux and the dielectric measurements with a close correlation between the heat flux and the dielectric constant. A slow increase and a sudden jump in the dielectric constant were observed in the heating process and at least five thermal anomalies were observed between 400.3 and 401.2 K in the cooling process. The unstable state was observed in the vicinity of Curie temperature keeping at a constant temperature both in the heating and cooling process. The hysteretic behaviors of the phase transition and the unstable state were discussed in terms of two-phase domains in the transition. (C) 2005 Elsevier B.V. All rights reserved.