In a game-based learning (GBL) environment for seventh-grade students, this study investigated the impact of competition, engagement in games, and the relationship between the two on students' in-game performance and flow experience, which, in turn, impacted their science learning outcomes. Structural equation modeling was employed to test a hypothesized path model. The findings showed that students' engagement in games not only predicted their in-game performance, but also had an impact on science learning outcomes via the mediation of in-game performance. While competition alone did not have a direct effect on either in-game performance or flow experience, it was indirectly related to in-game performance via the moderation of students' engagement in games. The study concludes with implications for future GBL interventions and studies.