Formation of anti-phase boundaries (APBs) in the Al5Ti3 long-period superstructure of Al-rich TiAl single crystals was examined by the high-resolution transmission electron microscopy (HRTEM) with image processing. The APBs were analyzed focusing on the unique units of ordered clusters such as Ti4Al, Ti3Al and Ti2Al which form square, fat rhombus and lean rhombus shapes in (002) Ti-rich layers, respectively. The Al5Ti3-type anti-phase domains (APDs) with various sizes in diameter were obtained in Ti-62.5at.%Al crystals by heat treatment at 600degreesC or 750degreesC after the floating zone melting and the subsequent quenching from 1200degreesC. Almost all the Al5Ti3-type APBs lay on {110) and were geometrically classified into 19 types named Type A to Type S. Indeed, 6 types of APBs (Type A to Type F) composed of the units of Ti4Al, Ti3Al and Ti2Al were found, but the other 13 types (Type G to Type S) were not clearly observed. Moreover, a specific type called Type A among the APBs was preferentially formed even in the initial stage of the APD growth. The APBs of Type A appeared with a quite higher frequency than those ideally formed by the random-nucleation and subsequent bounding of APDs. Thus, the APBs of Type A must be already introduced in the stage for nucleation of the APDs and grow stably on {110) due to the quite low APB energy in the long-period superstructure with a relatively large unit cell.