A new perovskite oxide, (Sr0.67Sm0.33)(Mn0.33Ti0.67)O-3, was synthesized at 1573 K in an Ar flow. The structure of the oxide was determined to be orthorhombic with the space group of Pnma using X-ray diffraction and electron diffraction. High-resolution transmission electron microscope (HRTEM) images of the oxide revealed that nano-domain structures were formed by twinning due to a phase transition. This is the first time that phase transition of the oxide has been observed in situ using a heating stage in a TEM. The high-temperature phase possessed an orthorhombic structure with the space group of Imma, which appeared above 723 K.