Graduate students regularly teach undergraduate STEM courses and can positively impact students' understanding of science. Yet little research examines graduate students' knowledge about nature of science (NOS) or instructional strategies for teaching graduate students about NOS. This exploratory study sought to understand how a 1-credit Teaching in Higher Education course that utilised an explicit, reflective, and mixed-context approach to NOS instruction impacted STEM graduate students' NOS conceptions and teaching intentions. Participants included 13 graduate students. Data sources included the Views of Nature of Science (VNOS-Form C) questionnaire administered pre- and post-instruction, semi-structured interviews with a subset of participants, and a NOS-related course project. Prior to instruction participants held many alternative NOS conceptions. Post-instruction, participants' NOS conceptions improved substantially, particularly in their understandings of theories and laws and the tentative nature of scientific knowledge. All 12 participants planning to teach NOS intended to use explicit instructional approaches. A majority of participants also integrated novel ideas to their intended NOS instruction. These results suggest that a teaching methods course for graduate students with embedded NOS instruction can address alternative NOS conceptions and facilitate intended use of effective NOS instruction. Future research understanding graduate students' NOS understandings and actual NOS instruction is warranted.