The microstructure of the damaged area created during a flash sintering event in BaTiO₃ was examined by high resolution transmission electron microscopy (HRTEM) and electron energy loss spectrometry (EELS) measurement. A DC electric field of 133 V/cm was applied to a fully-densified BaTiO₃ body prepared by conventional sintering, and the specimen temperature was elevated at a constant heating rate. The flash sintering event, at which the electric current in the specimen abruptly increases at the threshold field and temperature, took place at 890°C. After the flash event, tunnellike physical damage was observed in the direction of the field through the specimen. Formation of grain boundary second phase layers around the damaged area was confirmed by HRTEM observations. The grain boundary second layer was a crystalline phase with a low Ba/Ti atomic ratio of less than one. The temperature in some of the grain boundaries must increase by Joule heating during the flash event, thus the chemical composition in the vicinity of the grain boundaries changed due to vaporization of the Ba cations, resulting in the formation of the grain boundary second phase layers.