Hydrothermally synthesized barium titanate, BaTiO3 (commercially available), has internal pores in small particles. The three-dimensional structures of the pores were successfully observed using transmission electron microscopy (TEM) and reconstructed by a tomography system. The behavior of the internal pore is observed in situ with increasing temperature on the thermal stage of a TEM device. We succeed in recording the movement of pores and small grains. At >1128 K, some pores move out from the particle's surface during TEM observation. The temperature roughly agrees with the temperature at which the density of BaTiO3 powder sharply increases. During observation with increasing temperature, a thin layer appeared on the particle's surface at >73 K and then disappeared at 1193 K.