Background: A double-blinded, prospective, randomized, controlled study was designed to determine the intensity, duration of block, analgesic efficacy and tolerability of adding magnesium sulphate to intrathecal bupivacaine and fentanyl in partiuents scheduled for elective cesarean section.Patients and Methods: Sixty patients, age 18 to 45 years, undergo elective cesarean section under spinal anesthesia, ASA physical status I-II, singleton pregnancy and at least 36 weeks gestation. The selected patients were randomly divided into two Groups (A) control group and (B) intrathecal Mg group). Group (A) was received intrathecal 10mg (2ml) of 0.5% heavy bupivacaine plus 0.5ml (25µ g) fentanyl and 0.5ml saline (will addto make a total volume 3ml). While patients in Group (B) received (2ml) of 0.5% heavy bupivacaine plus 0.5mL (25µ g) fentanyl and Mg sulfate 0.75mg in 0.5ml (total volume 3ml) was injected intrathecally. Demographic data,clinical data, onset and duration of sensory and motor block and also complications (hypotension, nausea and vomiting) related to regional anesthesia were recorded. Numrical Rating Scale (NRS) was recorded every 6 hours for the next 24 hours. Rescue analgesia (ketorolac 30mg) was given when NRS was >4. Time of administration and total dose of rescue analgesia was calculated. The main results of our study showed that, there were significant increases in onset time and duration of sensory and motor block in Mg group. There were significant increase in time ofthe first dose of rescue analgesia in Mg groups than the control group. There were significant decrease in the number and dose of rescue analgesia in Mg groups than the control group.Conclusion: The addition of intrathecal magnesium sulfate to intrathecal bupivacaine plus fentanyl in patients undergoing cesarean section fasten the onset of anesthesia and prolongs its duration. Also, it prolongs the duration and quality of analgesia with reduction of the use of additional analgesia and lesser side effects.