Objective: The objective of this study is to compare the rates of depression and aggression among healthy siblings of 6-12 years old children with autism and Attention Deficit Hyperactivity Disorder. Method: The study is a survey. The statistical population consists of all healthy siblings of 6-12 years of age of the children with autism and Attention Deficit Hyperactivity Disorder in Tehran city. Seven siblings were chosen from each group, so the total sample size was 14. The research tool was the ASEBA test. Data were collected through the Child Behavior Checklist. T (P=0.05) test was used for data analysis.Results: The results showed that the rate of depression and aggression in siblings of autistic children is more than the siblings of the children with attention deficit hyperactivity disorder.Conclusion and recommendations: Given the fact that the amount of aggression and depression were higher among siblings of autistic children in comparison to the other group, training and consulting services should be available to the parents of autistic children in order to reduce or prevent aggression and depression among their not effected children.