【Objective】 To explore the diagnostic significance of P300 event related potential detect for brain cognitive dysfunction in children with attention deficit hyperactivity disorders. 【Methods】 The ADHD clinical subtypes, including predominantly inattention (PI), predominantly hyperactive-impulsive(HI) and combined type(CT), were classified according to the Fourth Edition of Diagnostic and Statistic Manual of Mental Disorder(DSM-IV) criteria. Sixty unmedicated ADHD children( ADHD-CT, n=20; ADHD-PI, n=20; ADHD-HI, n=20) with age range of 7~13 years were involved in the study, and 20 normal children match by age and gender were recruited as control. All participants were taken the P300 test with the 64-channel event related potential(ERP) and Net station 4.2, then contrast analysis of the latency, amplitude of P300 were made between the four groups. 【Results】 1)The latencies of P300 at forehead zone(Fz), central zone(Cz), left Cz(C3), right Cz(C4) and parietal zone(Pz) in ADHD groups were significantly longer than those in control group(P<0.05). 2)The amplitudes of P300 at Fz, Cz, C3, C4, Pz in ADHD groups were significantly lower than those in control group(P<0.05). 3)There was no difference between the three subtype groups(P>0.05). 【Conclusion】 ADHD has significantly impaired in brain cognitive function, but there is no difference between the three subtype groups.