Objective: Shivering during spinal anesthesia is a common complication in patients undergoing cesarean section.  In this prospective, randomized, double-blind study, we evaluated 10 and 20 mcg of intrathecally administered fentanyl would influence the incidence and severity of shivering in cesarean section patients. Methods: Ninety patients, ASA I scheduled for cesarean section under spinal anesthesia using hyperbaric 0.5% bupivacaine 2.0 ml with morphine 0.2 mg, were  randomly allocated to receive an additional 0.2 ml of fentanyl 10 mcg with normal saline 0.2 ml (group F 1), 0.4 ml of fentanyl 20 mcg (group F 2) or normal saline 0.4 ml (group S)Results: The incidence and severity of shivering after first hour in group F1 and group F2 were less than of in group S (P<0.05). There was no significant difference between the three groups regarding the incidence of hypotension, nausea, vomiting, pruritus, operative delivery, hypothermia, sedation, or respiratory depression. Conclusion: The addition of 10 mcg or 20 mcg fentanyl in morphine 0.2 mg and heavy bupivacaine 0.5 percent intrathecally had beneficial effect to reduce the incidence and severity of intraoperative and postoperative shivering after spinal anesthesia for cesarean section without increasing the other side effects.