In this study, we examined students' epistemological and positional framing during small group scientific modeling to explore their context-dependent perceptions about knowledge, themselves, and others. We focused on two small groups of Korean eighth-grade students who participated in six modeling activities about excretion. The two groups were purposefully selected because the students showed contrasting patterns of collaboration in modeling activities and sometimes showed significant shifts in their participation. We qualitatively analyzed the students' epistemological framing, positional framing, and modeling processes in each episode, based on their discourse and behaviors in the modeling practices. We found that the students' epistemological and positional framing were dynamically intertwined with each other and worked together to guide the students' participation in the group modeling. The students actively constructed and negotiated their perceptions about how to participate in the modeling practices, and those epistemological perceptions were closely related to how they positioned themselves and other participants in each context. This study suggests that it is important to attend to the dynamics of students' epistemological and positional framing in order to understand and facilitate the students' participation in science practice communities.