ADHD is one of the most prevalent psychiatric disorders in children.

ADHD have co-morbidity with many other psychiatric disorders.

Aim of this study was Comparing of prevalence of three subtypes of ADHD with psychiatric Co- morbidity in primary school students of Arak-Iran.

This study was a descriptive analytic, cross- sectional one, which carried on 2000 (960 girls and 1040 boys)primary school students in 1st state of Arak in academic year of 2009–2010, which screened by Intelligence Raiven scale and 48 Canner’s questionnaire (parents and teachers)and if there were existed of any subtypes of ADHD, we have done K-SADS-E questionnaire(Schedule for Affective disorder and Schizophrenia -epidemiological version)for them, then data were analyzed by using descriptive- statistical methods and Logistic- regression for obtaining R. R.

2.3, 3.5 and 2.7 percent of children have one of the subtypes of ADHD. The findings showed that prevalence of hyperactivity and attention deficit subtypes decreased while combined type increased by increasing age. The prevalence of two subtypes of hyperactivity and attention deficit in boys and girls were not significantly different but combined type was significantly different. Finally, there were differences between ADHD subtypes and Co morbidity of psychiatric disorders such as conduct, depressive,enuresis, tic, schizophrenia, oppositional, obsessive- compulsive, manic and social phobia, but these differences were not significant between girls and boys.

Prevalence of subtypes of the ADHD were different between age and gender but there were not significant differences between Co morbidity of ADHD subtypes and psychiatric disorders.