Research has demonstrated that compared to mastery-approach goals (i.e., trying to develop a mental ability), performance-approach goals (i.e., trying to outperform others) elicit higher metacognitive judgments, such as judgments of learning (JOLs). However, the underlying mechanism of the effects of achievement goals on JOLs remains unclear. Therefore, the present study conducted three experiments to examine the underlying mechanism of the effects of these goals on JOLs, based on the preexisting beliefs about achievement goals. The results of Experiment 1 showed that pre-study JOL in the performance-approach goal condition was higher than that in the mastery-approach goal condition, and Experiment 2 demonstrated that the difference of JOLs during learning (i.e., item-by-item JOLs) between the mastery-approach and performance-approach goal conditions remained constant from the beginning to end, suggesting that the beliefs about achievement goals affect item-by-item JOLs. In Experiment 3, participants read a description of a hypothetical experiment and estimated the performance of the group with mastery-approach and performance-approach goals. The results revealed that more participants believed that performance-approach goals have greater benefits than mastery-approach goals. Thus, the beliefs regarding achievement goals are one of the crucial factors affecting JOLs.