Objectives: This study was conducted to compare the efficacy of metacognitive therapy (MCT), fluvoxamine and the combination of MCT with fluvoxamine, in improving thought control strategies and stop signal criteria in patients with obsessive-compulsive disorder (OCD). Method: Twenty one individuals among outpatients with OCD presenting to clinics in Tehran were selected and randomly divided into three groups: metacognitive therapy, fluvoxamine (50-300 mg/d) and combined therapy. All groups received 10 days of treatment. The Thought Control Questionnaire (TCQ) and Stop Signal Questionnaire (SSQ) were administered pre-treatment and post-treatment to all subjects. Kruskal Wallis test, Mann-Whitney U-test, and ANCOVA were used to analyze data. Results: In comparison with fluvoxamine, MCT and combined treatment led to significant improvements in worry, self-punishment and reappraisal strategies of thought control and stop signal criteria (p<0.01). There was no significant difference between MCT and combined therapy. Conclusion: MCT and combined treatment are more effective than fluvoxamine (50-300 mg/d) in improving thought control strategies and stop signal criteria in patients with OCD. Adding medication to MCT does not improve the patients’ outcome.