This study assessed elementary school students' conceptions of STEM professions and its potential influence on STEM career interest, as well as the moderation effect of gender on the relationships between conceptions of STEM professionals and career interest in STEM. A total of 216 grade 3 through 5 students in Hong Kong participated in this study. They were asked to draw images of professionals in specific STEM areas, and complete questionnaires about their conceptions of STEM professionals and interests in STEM careers. Drawings were analyzed using graphic content analysis, and survey data were analyzed using moderated hierarchical regression analysis. Analysis of drawings showed that the students had inadequate understanding of engineers or scientists in the air, food, and water area. Boys were more likely to hold gender-related stereotypes about STEM professionals than girls. Analysis of survey data revealed that students' views about STEM career implications, STEM professionals' personal aptitudes, and social relations significantly predicted their STEM career interest. Gender significantly moderated the association of Social relations and Interest, meaning that compared to boys, girls might express higher STEM career interest when they more strongly believed that STEM professionals are able to build good social relationships with others. This study has implications for the design and implementation of pertinent lessons on STEM for elementary schools.