We aimed to investigate the inter- and intra-individual relationships between early adolescents' achievement goals, metacognitive monitoring, and academic outcomes. Korean sixth graders responded to achievement goal questionnaires, solved four quizzes, monitored their own confidence in these four quizzes, and took a final exam. Given the nested data structure, we conducted multi-level path modeling, which can distinguish between inter- and intra-individual effects in a single model. We found significant between-person indirect effects of mastery goals on the final exam score via individual's average scores for the four calibrations and for the four quizzes. We also found a positive within-person relationship between the calibration and quiz scores. The students' pursuit of personal mastery goals positively moderated this within-person relationship. These findings indicate that pursuing mastery goals can be more effective for metacognitive monitoring and knowledge acquisition, particularly for early adolescents.