Selected area and convergent-beam electron diffraction have been performed on LaGaO 3 at room temperature and 220 °C and LaGa 0.9 Mg 0.1 O 2.95 at room temperature. It has been experimentally established for the first time that the space groups of the orthorhombic LaGaO 3 at room temperature and rhombohedral one at 220 °C are Pbnm (No. 62) and R3¯c (No. 167), respectively. Crystal system of LaGa 0.9 Mg 0.1 O 2.95 at room temperature has been settled to be orthorhombic with space group of Pbnm (No. 62). Existence of a long-period anti-phase domain structure in LaGa 0.9 Mg 0.1 O 2.95 has been indicated from splitting and broadening of selected area electron diffraction spots.