Background and Purpose: Children's behavioral problems such as aggression can seriously harm children's social interactions. The aim of this study was to investigate the effectiveness of metacognitive therapy and attachment-based therapy on externalizing problems in children with aggression.

Method: The research method was semi-experimental with pre-test-post-test design with control group. The statistical population consisted of all aggressive male children in primary school in Isfahan during the school year 2015-2016 and their mothers. A sample of 45 of these children and their mothers was selected by available sampling method and randomly assigned to experimental and control groups. The mothers group received ten training sessions on attachment therapy and the children received 10 sessions of metacognitive therapy for 60 minutes. The instruments were aggression questionnaire (Bus & Preey, 1992) and children’s behavioral Inventory (Achenbakh, 2001). Data were analyzed using covariance analysis.

Results: The results showed that metacognitive therapy and attachment-based treatment were effective on externalized problems of male children with aggression (P  ≤  0.05). Also, based on the results of post hoc test, there was no significant difference between the effect of meta-cognitive therapy and attachment-based therapy (P  ≥  0.05).

Conclusion: Based on the findings of this study, meta-cognitive therapy and attachment therapy can be used to reduce the behavioral problems of aggressive children.