Background The use of opioids in intrathecal or epidural anesthesia has become popular to optimize postoperative analgesia. However, opioid-induced side effects, such as respiratory depression, nausea, vomiting, urinary retention and pruritus, limit their use. Objectives The purpose of this study was to assess the postoperative analgesic requirements and the analgesic effect of adding magnesium sulphate to epidural bupivacaine compared to addition of fentanyl in patients undergoing lower limb orthopedic surgery under combined spinal epidural anathesia. Patients and Methods After approval of ethical committee in our study and obtaining written informed consent from eligible patients the study was conducted on 60 patients classified to ASA I and II scheduled for lower limb orthopedic surgery. The study presents double armed randomized interventional prospective study including patients allocated into two equal groups each consists of 30 patients. Group (A): patients received magnesium sulphate added to bupivacaine in epidural, Group (B): patients received fentanyl added to bupivacaine in epidural Results The results of their study showed that the maximum level of sensory blockade was significantly higher in the combined fentanyl with magnesium group as compared with fentanyl group alone and magnesium sulphate group alone. The duration of sensory blockade of the combined fentanyl and magnesium group was significantly prolonged as compared to the other two groups as for the other two group the difference in the blockade duration was insignificant Conclusion We concluded that either magnesium sulfate (75 mg) combined with (10 ml) 0.25% Bupivacaine or fentanyl (1 µg/kg) combined with (10 ml) 0.25% Bupivacaine in combined spinal epidural improves intraoperative analgesia and prolongs early postoperative analgesia in lower limb orthopedic surgeries. The duration of analgesia was more prolonged in magnesium sulphate group than in the fentanyl group and this difference was statistically significant.