The purpose of this study was to develop a number theory textbook in order to improve prospective teachers' understanding of concepts and describe the feasibility of teaching materials number theory. The type of research used is the research development, with the development model chosen is a development model that fits the procedure suggested by Borg and Gall which consists of 5 steps. The steps taken are product analysis to be developed, initial product development, expert validation, and product revision, limited field trials, and product revisions, main field trials, and final products. Data creation techniques using product validation techniques and product trials. This study focuses on the steps of analyzing products that are developed up to expert validation steps. The validator suggests that textbooks are very good in all three aspects with an average score of 4.62 on a scale of 5 and can be continued at the Small-scale Trial stage.