Background & Objective: Obsessive– compulsive disorder is a disabling mental disorder that affects the quality of life, social and occupational functioning. Symptoms of obsessive– compulsive disorder are presence of both obsessions and compulsions. Obsessive– compulsive disorder is a complex and heterogeneous disease. If it is not treated, it will chronic. Possibility of recovery without treatment is low. Many treatments for this disorder are suggested including drug therapy, psychotherapy, family therapy, group therapy, electroconvulsive therapy (ECT) and psychosurgery. Today, cognitive– behavioral therapy is a selective method for treatment of obsessive– compulsive disorder. Exposure and response prevention (ERP) therapy is the main approach of cognitive– behavioral therapy for obsessive– compulsive disorder. In new cognitive theories, in the mode of anxiety disorders, metacognitive structures have a special place. Metacognition refers to beliefs and processes that are used to evaluate, adjustment or review and reflection of thoughts. Wells and Mathews have developed a model for obsessive– compulsive disorder, based on which obsessive thoughts engage in metacognitive beliefs related to the meaning of thoughts, and at the same time appeals to behavioral responses that can reduce the risk assessed with obsessive though. In the metacognitive approach, it focuses on revising beliefs about the importance and power of thoughts. The purpose of this study was to determine the effectiveness of metacognitive therapy in improving sign and metacognitive beliefs and thought fusion beliefs in patients with obsessive– compulsive disorder. Methods: In the present study we used non– congruent, multiple baseline experimental single case study design. Three women with obsessive– compulsive disorder (OCD) were selected from patients who referred to the counseling center of Zanjan university of medical science (Zanjan province, Northwest of Iran) in 2016٫ using purposeful sampling method, along with diagnostic interview and structured clinical interview for DSM– IV (First, Gibbon1997) based on disorders axis I. The efficacy of metacognitive therapy protocol was carried out in three phase of intervention (Baseline, 8 session treatment and 8 weeks follow– up). The data collection tools were the Y– Brown obsessive– compulsive inventory (Goodman 1989) has ten substances. The reliability of this scale is 0. 98 and the internal consistency coefficient of 0. 9 has been reported. Thought fusion questionnaire (Wells 2001), the questionnaire is a 14– item questionnaire. The Cronbach's alpha is 0. 89 and the convergent validity is 0. 65. Metacognition questionnaire (Wells 1997), this is a 30– item questionnaire whose reliability is 0. 87 and its Cronbach's alpha is 0. 93. Data analyzed with visuals inspection, improvement percentage, effect size and reliable change index (RCI) strategies. Results: According to the obtained results, the effect size of three variables measured was high than average. In addition, rate of reliable change index was high (1. 98) in all three variables, which is significant in clinical terms. Results showed metacognitive therapy was both clinically and statistically (p<0. 05) significant efficient on the reduction of treatment's target severity (signs of obsessive– compulsive disorder (OCD) and metacognitions and thought fusion). Conclusion: We conclude that metacognitive therapy was effective in improving the metacognitive beliefs and the thought fusion and the improvement of obsessive– compulsive symptoms. metacognitive therapy has appropriate efficacy in (OCD signs, metacognitive beliefs and thought fusion) reduction of people suffering from OCD.