Objective: This study attempted to determine whether the effects of physical exercise were reflected in the resting electroencephalographic (EEG) pattern of ADHD children. Method: Thirty-two ADHD children were assigned to either an exercise group or a control group. The exercise group participated in a water aerobics program for 8 weeks, whereas no intervention was administered to the control group. Resting EEGs were recorded under open-eyes condition before and after the intervention. Data from eligible participants, 15 from the exercise group (11 boys and 4 girls, 7.93 ± 1.02 years) and 14 from the control group (14 boys, 8.27 ± 1.04 years), were further analyzed. Results: While controlling for the baseline resting EEG, separate ANCOVAs indicated that the exercise group showed smaller theta/alpha ratios over the frontal and central brain sites after the intervention compared with the control group. Conclusion: This finding suggests that aerobic exercise may enhance the cognitive functions of children with ADHD, as reflected in resting EEG.