This is a randomized, single-blind, cross-over design study.Approximately 60 patients will be randomized to SSRIs or ERP group. Neuropsychological assessment and fMRI(Functional Magnetic Resonance Imaging） will be performed at baseline,during the treatment and at the end of the study. The research has two period. In the first period(before week 8) ,the patients will have monotherapy. In the second period(after week 8), rates of improvement based on the YBOCS(Yale-Brown Obsessive Compulsive Scale) scores will be assessed. The patient will be considered responsive to treatment when he or she has a reduction in YBOCS score ≥ 35% of the score at the baseline.If the patient is responsive to the treatment that he or she has received at the first 8 weeks,he or she will continue to follow the previous treatment strategy.If not,the patient will have a combination of SSRIs and ERP treatment for another 8 weeks. The follow-up period will last up to 6 months.The result of the study will improve our knowledge about the efficacy of the two first line treatments .Also,it will help us to find out the predictors of the outcome of the two different kinds of treatments.Strategy of clinical treatment of OCD will be more individualized in the future.