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.