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Polycrystalline La0.5Lu0.2Sr0.3MnO3 samples prepared by thermal decomposition are investigated by means of electron diffraction and high-resolution transmission electron microscopy. Besides rhombohedral La0.7Sr0.3MnO3 phase (R-phase) and hexagonal LuMnO3 phase (H-phase) reported by Huang, unexpectedly, an orthorhombic lattice (O-phase) with space group Pnma is observed in the interior of R-phase grain. The lattice parameters of the orthorhombic unit cell are a(o) = 5.44 angstrom, b(o) = 7.65 angstrom and c(o) = 5.48 angstrom The formation of O-phase results from a(-)b(+)a(-) type orthorhombic distortion of MnO6 octahedra induced by partial substitution of Lu3+ for La3+. Meanwhile, based on image simulation of the interface between R and O-phases, the atomic bonding on the interface is particularly discussed. (c) 2005 Elsevier B.V. All rights reserved.