Objectives: To determine the frequency of Mental retardation in children of attention deficit hyperactivity disorder. Study Design: A Cross-Sectional Study. Patient and Method: Study was conducted in the outpatient department (Clinics) of Sir Cowasjee Jahangir Institute of psychiatry Hyderabad. The study is of 06 months duration (1st January 2014 to 31st June 2014). Two hundred and fourteen (214) cases of ADHD age from 6years to 12 years were selected in this study. Non-probability consecutive sampling was done. Among patients with ADHD 6 months duration of symptoms either gender from age 6-12 years, attending Psychiatry clinic (OPD) at CJIP and LUH OPD were included in the study. Cases with Adverse General Medical Conditions, History of head injury, Head Injury, who deny/withdraw consent, were excluded. Data was collected on semi-structured Performa and analyzed through SPSS version 18 (PSW). Results: Out of 214 ADHD Children, mean age of the children was 9.50 years ±SD 2.32, in which 160 (74.8%) were males and 54 (25.2%) were females. n=105 (49.1%) of cases were from rural area and n=109 (50.9%) were from urban area. Co-morbid Mental retardation was present in 53.3% cases, from which 69.2% were male and 30.7% were female (p-value 0.034). 48.1% of children attended schooling despite 51.9% had not attended schooling (p-value 0.005). Conclusion: ADHD commonly present in male children. Co-morbid Mental retardation is significantly present in children with ADHD. Despite urban ruler difference Male gender significant associated with co-morbid Mental retardation.