Background: Shivering is an involuntary, repetitive contractions of skeletal muscles, which commonly occurred after spinal block and it is an uncomfortable problem to the patients and the anesthetists. Shivering is considered as a complication of anesthesia. Shivering has deleterious effects on the cardiac function especially in patients who have limited cardiopulmonary reserve and coronary disease, which could be explained by increased oxygen consumption, production of carbon dioxide and lactic acidosis caused by shivering. Objective: Our study was aiming at evaluating the effect of intrathecal nalbuphine versus intrathecal midazolam in the prevention of shivering during subarachnoid block. Patients and Methods: Ninety patients (ASA physical status I or II) scheduled for lower limb surgeries under spinal anesthesia were randomly allocated into three groups using sealed envelopes technique; Control group receiving mixture of bupivacaine and saline, Nalbuphine (N) group receiving nalbuphine and bupivacaine, and Midazolam group receiving midazolam and bupivacaine. Upon arrival to the operation room basic monitoring was applied and lactated ringer solution at room temperature was infused through peripheral venous catheter. Results: Shivering occurred in 20 patients (66.7%) in control group, 7 patients in nalbuphine group (23.3%), and 10 in midazolam group (33.3%). The incidence of shivering and core temperature differed significantly between group N and the other two groups (P values in saline and midazolam groups > 0.05, while that of nalbuphine < 0.05). Conclusion: Intrathecal nalbuphine is more effective than intrathecal midazolam in prevention of post-spinal shivering for patients undergoing lower limb surgery.