It has been suggested that students' epistemic understandings influence their practice of argumentation. However, how that influence takes place is not yet fully explained. To contribute to the answer to this question, this study explored elementary students' epistemic understandings in their scientific argumentation. A 4-month qualitative case study with descriptive and interpretive emphases was conducted in a grade 5/6 science classroom in Canada. Nineteen students and their teacher participated in this study. Multiple methods for data collection were employed, including observation, interviews, and collecting students' works. It was found that these students were aware of their epistemic understandings. Qualitative data analysis also revealed that students' epistemic understandings can, yet do not always, influence their performance in argumentation by affecting the ways they think and do while engaged in argumentative dialogues. Moreover, it was found that elementary students were capable of refining their epistemic understandings through social interactions with peers and adapting their epistemic understandings in new contexts. Based on the findings, instructional suggestions are discussed to support the development of students' epistemic understandings and argumentation skills.