The purpose of this study is to explore the ways of unit construction in Korean textbook. This study analyzed how the reading domain is integrated with other domains in 23 authorized textbooks for the first grade in the middle school. Most reading units are proved as domain integration units. The achievement standards of reading domain are integrated with other domains 16 to 27 times in the reading units in the 23 authorized textbooks. Writing domain is the most frequently integrated with reading domain. Speaking and grammar are the next. The way of integration of main units is arrangement, combination, combination after arrangement and arrangement after combination. The arrangement is that domains are integrated in the main units and each content of domain consists the small units. The combination is that domains are integrated in the small units. In the arrangement, the small unit’s objective should be presented so that teachers and students can recognize it clearly because the main unit’s objective does not match the small unit’s one. The introduction of the way of integration is necessary in the curriculum level for the efficiency of teaching and learning in the lesson since the change in the content of the curriculum happens in the domain integration.