High-quality Sr2FeMoO6 (SFMO) was achieved and examined by X-ray analysis via the Rietveld structure refinement. Detail structure of anti-phase boundaries (APBs) in SFMO was investigated by means of high-resolution transmission electron microscopy. We discovered that two types of APBs were simultaneously present inside core-shell grains, wherein high-density maze-type APBs were in the core and swirl-type APBs were in the shell part. It is highlighted that the atom arrangements of structures of both types of APBs are confirmed to be zigzag. Underlying present discovery and the formation mechanism of SFMO, a mechanism concerning development of APBs in SFMO was proposed, which clarifies not only the simultaneous existing of maze-and swirl-type APBs but also their related morphology formation. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.043203jes] All rights reserved.