The present study examines the effects of detailed diagrams on comprehension monitoring accuracy. The participants consisted of 45 university students who were asked to study six human anatomy texts with either detailed or simplified diagrams, judge how well they understood each text, and complete the tests for each text. The participants also rated their emotions before and after the texts. Comprehension monitoring accuracy was computed as the intra-individual correlation between judgments and test performance. The results showed that participants who had learned with detailed diagrams were less accurate in judging their comprehension than those who had learned with simplified diagrams. Positive emotions significantly decreased after learning with detailed diagrams, whereas it did not change significantly after learning with simplified diagrams. These findings support the idea that adding unnecessary details in diagrams may lead students to rely on invalid cues for assessing their own level of comprehension, thus resulting in poor monitoring accuracy.