The purpose of this study is to compare readabilities of elementary school textbooks and to draw suggestions in developing textbooks. For this, I compare among 3th grade's korean language textbook, social science textbook, science textbook. The vocabulary difficulty is decided by average grade of words and the sentence difficulty is decided by average numbers of words per sentence. In vocabulary level, the readability of korean language textbook is the highest and the readability of social science textbook is the lowest. And in sentence level, the readability of korean language textbook is the highest and the readability of science textbook is the lowest. So, when we develop content area textbooks, we should make effort to raise their readabilities through simplifying and elaborating. For this, we have to study first the readabilities of textbooks on each subject and on each grade. And we have to prepare the system for monitoring and controlling the textbooks' readabilities.