Background: The aim of this study was to evaluate the effect of metacognitive therapy (MCT) on metacognitive beliefs, metaworry, and the signs and symptoms of patients with generalized anxiety disorder (GAD).Materials and Methods: In this study, all patients with GAD referred to the psychiatric clinic of Baghiyatallah hospital (during 2012-2013) were selected and assigned into the four (MCT, combined treatment, pharmacotherapy and control) groups. Patients under MCT and other patients were evaluated 6 and 4 times, respectively by metacognitive (MCQ-30) and metaworry questionnaires (MWQ), Beck anxiety inventory (BAI), Beck depression inventory (BDI-II) and Hamilton anxiety rating scale (HARS). Data were analyzed using the indices for trend changes, slope, Cohen rate of variability, interpretation of the changes of variables in the graphs; the recovery percent and effect size were used to determine the clinical significance.Results: MCT and combined treatment (in post-treatment and follow-up phases) significantly decreased the metacognitive beliefs and metaworry in the patients received MCT and also could help to control the patients' anxiety and depressive signs and symptoms. Moreover, pharmacotherapy had only effect on the control of anxiety and depressive signs and symptoms in post-treatment, but after the termination of pharmacotherapy, the patients to some degrees experienced a recurrence of the signs and symptoms.Conclusion: MCT (with or without pharmacotherapy) shows high efficacy during less than half of the sessions of constitutive therapies (e.g. cognitive/behavioral therapy) in terms of controlling the underlying mechanisms of pathological anxiety.