Since the 1990’s, many researchers have clarified that children with Attention DeficitHyperactivity Disorder (ADHD) had unrealistically positive self-perception (positive illusorybias; PIB). That is, children with ADHD consistently have shown high self-perception despitethe fact that they could only achieve low performance. For the mechanism of the PIB, someexplanations have been made, such as cognitive immature, neuropsychological deficits,ignorance of incompetence, or self-protection. Additionally, it has been suggested thatcomorbid disorders (depression, aggression, or academic difficulties) or subtype(predominantly inattentive type or predominantly hyperactive/impulsive type) might modifythe PIB in ADHD. The author pointed out the symptom similarities between bipolar disorder(BD) and ADHD, so that some children with BD might have misdiagnosed as ADHD, andBD might take over the PIB in ADHD. Finally, the future investigation should include BD tounfold the PIB in ADHD and is recommended analogue study on the PIB.