The objective of this study was to investigate the brain mechanism involved in the regulation of impulsivity in children with Attention Deficit and Hyperactivity Disorder (ADHD) through error detection as well as error monitoring. The subjects in this study included 7-11-year-old impulsive ADHD children as well as normal children and adult controls. Error related negativity (ERN) and error positivity (Pe) were measured. ERN peak latency from the children groups was delayed significantly when compared with the adult group; however, no significant difference in ERN amplitude was found among the three groups. Impulsive ADHD children had the earliest peak latency of Pe. In addition, the average Pe amplitude in impulsive children was significantly smaller than in adults (Cz and Pz), and smaller than in normal children (Pz). Late conscious cognitive processing of error is significantly weaker in impulsive ADHD children, suggesting a serious deficit of late error monitoring, rather than error detection.