This study assessed the role of memory beliefs about font size in judgments of learning by examining the effect of font size on pre-judgments of learning and judgments of learning. The results showed that: (1) The font size affected the judgment of learning, but had no significant effect on the recall performance. (2) Individuals have the belief that large words were easier to remember than small words. (3) Compared with the pre-judgments of learning based on memory belief, the level of learning judgment made by individuals based on font size cues shows a significant decline. (4) The time it takes for individuals to make pre-judgments of learning based on large-font cues is significantly less than that for individuals to make pre-judgments of learning based on small-font cues. These results suggest that the font size effect of judgments of learning is a phenomenon of metacognitive illusion, and belief can explain to some extent the variability of the font size effect in judgments of learning, which partly supports the belief hypothesis.