This research is purposed to compare cognitive functions between normal, ADHD and RD children. We selected three groups of 30 primary school students (normal, ADHD and RD) by available sampling. These groups were matched in age (8-9), IQ (96-116) and gender (Female and Male). We evaluated their brain cognitive functions by CAS (cognitive assessment system). The results showed that normal group's performances (Full Scale) were meaningfully better than ADHD and RD groups but, there were no meaningful differences between ADHD and RD subject groups. Also Normal children's performance in all four scales of PASS was meaningfully better than ADHD and RD children, but there was no meaningful difference between the average scores of four scales between ADHD and RD children.