Science identity has been widely discussed in recent years; however, research on its development in multimodal composing environments, especially in formal classroom settings, has yet to be fully investigated. This qualitative study unraveled the science identity development of sixth-grade students as they created multimodal science fiction stories in a STEAM course. Thirty-two students enrolled in the course and worked in groups of 3-5, and each student self-selected one of three roles: designer, scientist, or writer. This study focused on the students (n = 9) who took the role of scientist and examined their science identity development. Data sources include digital surveys, semi-structured group interviews, and multimodal artifacts. Our qualitative analysis suggests that (a) composing with modes of choices could drive interests in science; (b) students connected science practices in classrooms with those in professional domains through taking the role of scientist; (c) taking hybrid roles (i.e., a combination of scientist and other roles) while composing with multiple modes contributed to the recognition of science in non-science careers. Based on these findings, we discuss the implications for cultivating positive science identities and engaging early adolescents in career exploration.