In Vietnamese schools, textbooks used in schools were compiled by authors whom were chosen  by the Ministry of Education and Training; both teachers and students used the same textbooks in process of teaching and learning. Particularly in mathematics education of Vietnam, textbooks of mathematics always are main materials which each school student was equipped to support the process of learning both at home and at classroom.  In order to know how the mathematics teachers in schools have used textbooks in their teaching, we carried out an investigation of secondary school teachers of mathematics with two research questions as follows: (1) How have the secondary teachers of mathematics used textbooks in their teaching? (2) Are there any differences between junior school teachers of mathematics and senior school teachers of mathematics in their use of textbook? The results show that  all stages of a process of teaching a mathematics content such as making lesson plan, designing learning activities of students in the classroom, giving exercises to students, all teachers based on textbooks.