<p>With the rapid development of science, technology, engineering, and mathematics (STEM) education, more and more primary and middle school students have begun to receive preliminary STEM courses. However, the systematic instrument for measuring younger students’ science self-efficacy and motivation in Mainland China has not been well developed, particularly for implementing STEM education at the initial stage. Based on this background, this study developed a science self-efficacy and motivation instrument for primary and middle schools in Mainland China. The measured science self-efficacy and motivation may be representative of students’ STEM self-efficacy and motivation at the beginning stage of STEM education. There were 614 Mainland Chinese students who participated in this study. Through the processes of the initial item selection, teacher-student interviews, and pilot test, the formal questionnaire was designed and developed. The results of the confirmatory factor analysis (CFA) indicated that the instrument was suitable for younger students in Mainland China. Based on this, the independent sample <i>t</i>-test and the one-way ANOVA analysis showed that there was no significant gender difference, while the grade difference was significant. Students’ science motivation and self-efficacy increase with grade level. In addition, we offer some insights for future research based on these results.</p>