The synthesis mechanism and microstructures of BaIn2O4 particles were analyzed by simultaneous thermogravimetry - differential scanning calorimetry (TG-DSC), X-ray diffraction (XRD), and scanning electron microscope (SEM). Firstly, In(OH)3 and BaCO3 precursors were prepared by the co-precipitation method. Next, during the sintering process In(OH)3 initially decomposed into In2O3 and water, and then BaCO3 reacted with In2O3 to synthesize Ba4In6O13. Finally, Ba4In6O13 and In2O3 further reacted to form BaIn2O4. The obtained BaIn2O4 particles were in monoclinic structure and exhibited high crystal quality. The grains were tightly bound together and their boundaries became blurry. The grain sizes increased with increasing the sintering temperature.