This study examined the formation of BaTiO3 from degassed barium titanyl oxalate (degassed BTO), which was heat treated at 500°C for 2 h followed by 20 min of aqueous milling, upon further heating. The evolution of BaCO3, Ti‐based interphase and BaTiO3 at various temperatures was examined by X‐ray diffraction, high‐resolution scanning transmission electron microscopy, and electron energy loss spectroscopy. When the degassed BTO was exposed to an aqueous milling, significant amounts of needle‐shaped BaCO3 crystals formed preferentially, which made this system similar to that of a solid‐state reaction. However, the formation mechanism of BaTiO3 observed in this study was found to be different from that of a solid‐state reaction due to the different nature of the Ti‐based phase.