Abstract: This study investigated how teachers develop their knowledge of instructional materials. The research data were collected from 77 mathematics teachers in three best high schools in the metropolitan area of Chicago, U. S. through a questionnaire survey, classroom observation and teacher interview. The findings of this knowledge of instructional materials. Overall, teachers’ own teaching experience and reflection, and their daily exchanges with their colleagues are the most important sources; teachers’ attending organized professional activities, inservice training and experience as school students are the secondarily important sources; and their reading professional journals and books, and preservice training are the least important ones. Statistically, the contributions of those three kinds of sources to the development of teachers’ knowledge of instructional materials are significantly different.