This study is aimed to evaluate the effect of adding MgSO4 to intrathecal marcain to control shivering during cesarean section surgery with spinal anesthesia. Sixty patients, ASA physical status I or II, aged between 18 - 35 years, undergoing cesarean surgery in Rasoul Hospital during 2010 to 2011, are included in this randomized, prospective, double-blind and placebo-controlled study. Patients are randomized into two groups of 30 individuals. The control group receives normal saline and treatment group receives magnesium sulfate. The treatment group receives 2 cc of marcain 0.5% (10 mg) and 0.5 cc of magnesium sulfate 5% (25 mg) and control group receives 2 cc of marcain 0.5% and 0.5 cc of normal saline. Body temperature will be recorded before spinal block, immediately after spinal block and each 30 mine during 1 hour after admission to recovery. Indeed, vital sign, 5 minutes for 15 minutes then each 10 minutes till the end of recovery will be recorded. Sedation will be recorded, before spinal block, immediately after spinal block, each 30 minutes during 1 hour after admission to recovery. Shivering will be recorded during 90 minutes after spinal block in 0, 5, 10, 15, 20, 30, 40, 50, 60 and 90 minutes by physician who is unaware of the type of treatment given to the patients.