In-situ observations using high-temperature transmission electron microscopy (TEM) is a promising technique for obtaining new findings and developing phenomenological theory for ceramic materials at high temperatures. By observing the changes in hydrothermal BaTiO₃ due to heating, we clarify the vanishing mechanism of internal pores and the mechanism of BaCO₃ phase generation/vanishing at high temperatures. Through observations of the grain growth of BaTiO₃ prepared by radio-frequency plasma chemical vapor deposition, we calculate the grain-boundary diffusion coefficient, which is an important parameter controlling the sinterability of ceramics. Observations of the shrinkage and disappearance of Ba₅Nb₄O₁₅, as well as simulations, indicate that high energy and low diffusivity at the grain boundary make grains vanish while maintaining a truncated spherical shape. Finally, observations of the structural changes in layered perovskite BaLn₂Mn₂O₇ (Ln = Gd, Pr, Eu) reveal a first-order phase transition that has not been previously reported.