The impact of Augmented Reality (AR)-based science learning on cognitive development has been established, but the effect of AR on the improvement of students' academic performance remains inconclusive. Additionally, while epistemic beliefs as a significant determinant on student learning outcomes is well-documented, there is little research in AR-supported science learning exploring the effectiveness of AR activities on students' epistemic beliefs. This mixed-methods study investigates the relationship between students' engagement in an AR-based inquiry learning environment and their epistemic beliefs. It aims to examine if students' engagement in AR activities has an impact on their epistemic beliefs and academic performance, as well as which aspects of students' epistemic beliefs may be affected by the designed activities. 159 fifth-grade students participated in the AR activities for two months. The findings revealed that students' academic performance improved significantly, evidenced in the pre- and post-tests. While students' perceived engagement did not significantly affect their academic performance, students' perceived engagement in the activities did influence their epistemic beliefs, particularly in the dimension of justification of knowledge in science. The study identified features of learners' interactions in the AR learning environment and these findings provide insights into potential areas for improvement in AR-based science learning.