[Purpose] The aim of this study was to investigate the P300 in electroencephalography (EEG) and the reaction time in children with attention deficit hyperactivity disorder (ADHD) during an auditory oddball task when sitting on a classroom chair or therapy ball. [Subjects] Fifteen ADHD children with a mean age of 8.6 ± 2.1 years and 14 healthy children with a mean age of 8.7 ± 2.0 years were used as subjects in this study. [Methods] All subjects were asked to sit on a chair or therapy ball and perform simultaneously the auditory oddball task. A portable 40-channel EEG system and a sound operating system were employed to record and analyze the EEG and button reaction time signals. [Results] When seated on the chair, the ADHD group had a significantly longer reaction time than the control group. ADHD children seated on a therapy ball showed a significant improvement in reaction time compared with when seated on the chair. In the parietal lobe, the ADHD group had a significantly delayed P300 latency during chair seating compared with the control group. The ADHD group showed a significantly shorter P300 latency time when seated on a therapy ball. [Conclusion] The therapy ball has a significant advantage for enhancing the attentional ability in children with ADHD.