In order to investigate the developmental changes of frontal lobe functions, frontal assessment battery at bedside (FAB) and event related potentials (ERPs) during a Go/NoGo task were collected. In addition, we compared FAB score and ERPs in AD/HD patients with that of normal children. The total scores of FAB in normal subjects dramatically increased at around 10years old and reached to full scores at around 20years old. The scores with AD/HD were significantly decreased compared to normal children. We obtained ERPs of the Go and NoGo conditions from EEG recordings during a visual Go/NoGo task and measured peak latency and amplitude of the NoGo-N2 and NoGo-P3 components. In normal children, the NoGo-N2 amplitude diminished and the NoGo-P3 amplitude increased linearly with age. The pattern of NoGo-N2 and NoGo-P3 components in 7- to 15-year-old children were similar, but differed in adults. In AD/HD patients ranging in age from 5 to 9, the NoGo-N2 amplitude decreased and the NoGo-P3 latency was prolonged compared to age-matched control subjects. These results suggested that the maturation of response inhibition in frontal lobe is achieved beyond 15years of age and that the function of response inhibition is immature in AD/HD patients.