This is an open-label (all people know the identity of the intervention), prospective (study following participants forward in time), self-controlled, long-term post-marketing study in participants with ADHD. The study consists of 3 periods: screening period, wash-out period/run-in period (3 days), and open-label treatment period (12 weeks) which is divided into 2 periods that is dosage adjustment period and dose-optimization period. The total duration of the study is 12 weeks. During the dosage adjustment period, the dose of methylphenidate hydrochloride was adjusted to the optimal dose within 1-3 weeks starting with initial dosage of 18 milligram per day (mg/d) which can be increased to 36 mg/d up to a maximum dosage of 54 mg/d according to the therapeutic effect and tolerance or maintained at 36 mg or re-adjusted to 18 mg due to intolerance. Participants will receive the optimal dose determined during dose adjustment period for 9 weeks during the dose-optimization period. In the study, 2 types of participants will be enrolled in 2 groups (according to condition): OROS-MPH group (participants with ADHD) and the normal children group (healthy participants). The efficacy will be assessed primarily by Digit Span Test and Inattention /Over activity With Aggression (IOWA) Conners evaluation scale. Participants' safety will be monitored throughout the study.