The purposes of this study were (1) to develop a questionnaire measuring high school students' conceptions of the relationship between mathematics and physics, (2) and to determine the students' conceptions of the relationship between mathematics and physics. A total of 718 high school students (343 male, 375 female) participated in this study. Espousing a phenomenographic approach, a Relationship between Mathematics and Physics Questionnaire (RMPQ) was first developed. Then explanatory factor analysis (EFA) to identify factor structure and confirmatory factor analyses (CFA) to verify the EFA results were performed on the data obtained. Data analyses showed that the RMPQ was a reliable and valid instrument to identify the students' conceptions of the relationship between mathematics and physics. Descriptive results also revealed that the majority of the students believed that mathematics and physics closely related to each other in terms of their content and daily-life relations. They also believed that mathematics was needed to understand physics.