Background: Surgical and laparoscopic techniques are two different methods for the removal of gall bladder. Today, laparoscopic cholecystectomy is a preferred method for short-term hospitalization and early return to function related to minimal invasive surgical technique. However, patients still complain of significant postoperative pain, secondary inflammation of the diaphragm and the nociceptive genus of the annoying membrane's peritoneum. Multimodal analgesia is necessary for managing pain after laparoscopic cholecystectomy. Magnesium sulfate is a new emerging medication for the management of acute pain. There are no previous reports to compare the analgesic effect of intraperitoneal instillation of bupivacaine plus morphine hydrochloride and bupivacaine plus magnesium sulfate for postoperative pain after laparoscopic cholecystectomy. Aim: The purpose of this study is to compare the analgesic effect of intraperitoneal instillation of bupivacaine plus morphine hydrochloride versus bupivacaine plus magnesium sulfate in patients undergoing laparoscopic cholecystectomy under general anesthesia for better pain relief and less opioid consumption during the first 24 hours. Methods: Following the approval of the Institutional Review Board of An-Najah National University and written informed consent from patients undergoing laparoscopic cholecystectomy, hundred patients between 18 and 60 years old, American Society of Anesthesiologist (ASA) Grades I and II, were randomized to one of the following groups by the sealed envelope: (Mo group) (n=50) receiving intraperitoneal instillation of 30 ml 0.25% bupivacaine and 3 mg morphine and (Mg group) (n=50) receiving intraperitoneal instillation of 0.25% bupivacaine plus 50 mg/kg magnesium sulfate to a total volume of 30 ml. Medications were given after peritoneal wash and suctioning through intraperitoneal instillation. A drug solution is prepared by a doctor who does not participate in the study. All patients received the same anesthesia method, general anesthesia was administered. The induction protocol was standard for all patients. Patients were monitored for electrocardiogram (ECG), heart rate, blood oxygenation (SpO 2 %) and noninvasive blood pressure (NIBP). Postoperative pain was evaluated using visual analog scale (pain score of 0-10). The participants were evaluated for 24 hours after the operation with the registration of abdominal pain. The postoperative pain outcome was reported at 0 and 30 min, 1, 4, 8, 12, 16 and 24 hours. The cutoff value for VAS is 4 for indication of rescue medication. At VAS ≥ 4, rescue analgesics were administered on request (20 mg of pethidine) intravenously in Post Anesthetic Care Unit (PACU) and 50 mg intramuscularly in the surgical ward.