Objective. The present study examined whether obsessive beliefs change over time in the OCD patients receiving selective serotonin reuptake inhibitors (SSRIs) and the impact of obsessive beliefs in treatment response. Methods. In the first part of a two-stage study comparing the efficacy of antipsychotics as augmenting agent in SSRI-resistant OCD patients, 57 patients were interviewed with the Yale-Brown Obsessive Compulsive Scale (Y-BOCS), Hamilton Depression Rating Scale (HDRS) and Obsessive Beliefs Questionnaire-44 (OBQ-44) before and after 12-week of SSRI treatment period. Results. All OBQ-44 subscale scores significantly decreased with SSRI treatment. The mean changes in OBQ-44 Importance and Control of Intrusive Thoughts (I/CT) subscale and HDRS total scores of responders were significantly higher than those of SSRI-resistant patients. The baseline OBQ-44 P/C and Y-BOCS obsession subscale scores significantly predicted the treatment resistance in a logistic regression model. Conclusions. The alleviation of negative mood by SSRIs may help the sufferer to disengage from dysfunctional appraisals. Since individuals with highly obsessive beliefs about P/C are more likely to be resistant to SSRI treatment, the treatment of OCD can be made more effective when focusing on altering appraisals about P/C.