Background: Obsessive-compulsive disorder (OCD) is the fourth common psychiatric disorder. Among the anxiety disorders, OCD has the least therapeutic response and 40-60% of OCD patients do not satisfactorily respond to the first-line standard treatment known as treatment-resistant OCD. One of the best therapeutic strategies is the augmentation therapy, which is adding antipsychotics to the standard treatment (SSRIs). Objectives: This study was aimed at comparing the efficacy of Risperidone and Aripiprazole as augmentation therapy in the resistant cases of obsessive-compulsive disorder. Methods: In this double-blind randomized clinical trial, 100 patients with treatment-resistant OCD were diagnosed based on the DSM-IV-TR and were followed for 12-weeks. The patients were randomly divided into two groups of Aripiprazole and Risperidone and received an average daily dose of 5 mg and 1.5 mg for twelve weeks, respectively. The efficacy of treatment was measured and compared by the Yale-brown obsessive-compulsive scale (Y-BOCS) at 4, 8 and 12 weeks. Results: The mean Y-BOCS score of patients in Risperidone and Aripiprazole groups were 25.26 ± 4.17 and 25.02 ± 4.46; respectively and had no significant difference (P = 0.79) at the beginning of the trial. At the end of the study (12 th week), it was changed for the Risperidone and Aripiprazole groups to 20.00 ± 4.45 and 16.24 ± 4.41, respectively (P < 0.001). Furthermore, there was a significant decreasing trend of Y-BOCS scores in both groups, which was demonstrated by the repeated measurement analysis (P < 00.1). Conclusions: It was found that both Aripiprazole and Risperidone could be effective in the treatment of treatment-resistant OCD patients. However, Aripiprazole showed a higher efficacy compared to Risperidone.