Background: The underlying mechanism of time perception deficit in long time intervals in attention-deficit/hyperactivity disorder (ADHD) is still unclear. This study used the time reproduction dual task to explore the role of the attentional resource in time perception deficits among children and adolescents with ADHD. Methods: Participants included 168 children and adolescents with DSM-IV ADHD and 90 control children and adolescents without ADHD, aged 10 to 17 years, in Taipei. The DSM-IV diagnoses of ADHD and other psychiatric comorbid conditions were made by clinical assessments and confirmed by the psychiatric interviews of both parents and participants using the Chinese Kiddie Epidemiologic version of the Schedule for Affective Disorders and Schizophrenia. The participants were also assessed by using the Wechsler Intelligence Scale for Children-3rd edition (WISC-III), and time reproduction tasks (the single task and the simple and difficult versions of the dual tasks) at 5-second, 12-second, and 17-second intervals. The linear mixed model was used for data analysis. Results: Children and adolescents with ADHD had less precise time reproduction than the controls in all three tasks except the 5-second interval of the single task. There were significant interactions between group and interval (12-second vs. 5-second, p = 0.030; 17-second vs. 5-second, p less than 0.001), and between group and task (simple dual task vs. single task, p = 0.016; difficult dual task vs. single task, p less than 0.001) after controlling for FSIQ, comorbidity, sex, age, use of methylphenidate, and the performance of the non-temporal tasks in dual tasks, if relevant. Conclusions: Significantly increased estimation errors in ADHD with increased task difficulties suggest that impaired timing processing in children and adolescents with ADHD during long time intervals may be accounted for by the limited attentional capacity rather than a primary problem in timing per se. This finding does not apply to rapid time intervals, in which cerebellar circuitry is important.