Attention Deficit Hyperactivity Disorder is the most common neuro- developmental disorders of childhood characterized by the core symptoms including inattentiveness and distractibility and frequently involve impairments in executive functioning, increased impulsivity, and restlessness. Objective To find out the prevalence of Attention Deficit Hyperactivity Disorder among school aged children Presented to Pediatric clinic and also to investigate associated comorbidities. Method This study was conducted at Nobel Medical College Teaching Hospital, Biratnagar during April 2014 – March 2015. Attention Deficit Hyperactivity Disorder was diagnosed by the developmental Pediatrician after taking relevant history and the clinical assessment using Attention Deficit Hyperactivity Disorder rating scale with diagnostic criteria consistent with Diagnostic Statistical Manual – IV classification. Spence anxiety scale child and parent rated version and Strength and Difficulty Questionnaire were also used. Result Result showed the yearly prevalence of Attention Deficit Hyperactivity Disorder being 41(11.7%) with male: female ratio of 4:1. The study reported that Attention Deficit Hyperactivity Disorder combined type was the commonest type that was 26(63%) cases followed by Inattentive type 9(22%) cases and 6(15%) were hyperactive type. The mean age for Attention Deficit Hyperactivity Disorder was 7 years and 5 months. The most common co-morbidities were sleep problem 12(29.3%), Learning difficulty 10(24.4%), Anxiety disorder 10(24.4), Oppositional Defiant Disorder 9(22%), Autism Spectrum Disorder 5(12%), speech delay 6(14.6%), and 4(10%) had associated tics. There was abnormal SDQ prevalence of 29.3% across the area of emotional distress. The mean abnormal SDQ score in total difficulty area 8 (20.7%), socializing with peer 9(22%), behavioral difficulty 11(26.8%), hyperactivity/inattention 23(56.1%) and impact of difficulties in young person’s life being 5(12.2%). Conclusion There is high prevalence of Attention Deficit Hyperactivity Disorder in children and adolescents in Nepal. This study has also looked at co-morbidities associated with Attention Deficit Hyperactivity Disorder and the findings of the study have thrown lights on the mental health and other developmental issues associated with it.