Developing students’ interest in STEM careers in STEM education is important. This study explored the impacts of STEM instruction involving scientists or engineers on 224 Hong Kong students’ interests in STEM careers and their stereotypes about STEM professionals. The teachers who participated in professional development, namely “School-STEM professional collaboration”, implemented the STEM instruction with support from educational researchers and invited STEM experts. Throughout the STEM instruction, students were provided with STEM role models, and were engaged in authentic and hands-on activities to mimic the research work of invited STEM experts. Data were collected through pre- and post-surveys and were analysed using descriptive statistics, t tests, and ANOVAs. The results revealed increased interest in STEM careers, and more positive perceptions of STEM professionals among the students after the instruction. In addition, the impacts of the instruction were found to be influenced by gender-matching between students and STEM professionals. Girls were more likely to improve their interests and alter stereotypes with exposure to female role models. It can be concluded that the STEM instruction diversifying the stereotypes of STEM people can be beneficial to students. This study has implications for supporting more effective STEM education in primary schools. Copyright © 2020 National Institute of Education, Singapore.