Science, Technology, Engineering, and Mathematics (STEM) education has become one of the main priorities in the United States. Science education communities and researchers advocate for integration of STEM disciplines throughout the teaching curriculum. This requires teacher knowledge in STEM disciplines, as well as competence in scientific literacy. Since nature of science (NOS) is a critical component of scientific literacy, this study examined teachers' conceptions NOS over a one-year period. Participants included 21 middle school science and mathematics teachers who integrated science and mathematics in their classrooms. We employed two NOS instruments to collect data on participants' NOS conceptions "before" and "after" a one-year online graduate program. This study examined changes in NOS understanding for the group as a whole, between science and mathematics teachers, and whether beginning and experienced teachers differed in their conceptions of NOS. Findings show that the teachers' conceptions of NOS improved significantly after two semesters of explicit, reflective NOS instruction. There was no significant difference between science and math teachers' conceptions of NOS. The notion that science teachers know just as much about NOS as mathematics teachers indicates that science teachers in the U.S. are just as unfamiliar with the nature of science as mathematics teachers. In addition, years of experience did not play a role in the participants? conceptions of NOS. Examination of teachers' conceptions of NOS will help researchers, teacher educators and teacher professional development providers gain insight on ways to develop STEM teachers' conceptions of NOS.