Background: Study of effective modality for postoperative pain management has remained a subject of ongoing clinical researches due to its uniqueness and associated complex physiological consequences with somatic, autonomic and behavioral manifestations. Poorly managed postoperative pain can also increase the incidence of persistent postoperative pain conditions. Objectives: To compare between the effect of intravenous and intraperitoneal magnesium sulphate on postoperative pain outcomes, post-operative opioid consumption as well as intraoperative fentanyl consumption in patients undergoing laparoscopic cholecystectomy. Patients and Method: This prospective randomized double blind clinical study was carried out on 80 adult patients, Undergoing laparoscopic cholecyctectomy at Al-Azhar University Hospitals. After approval by the Institutional Ethical Committee, and informed written consents were obtained from the patients. We randomly divided the patients into two equal groups: Group A received Magnesium sulphate (magnisol 50 mg/kg in 250 ml of isotonic 0.9% normal saline) intravenously over 30 minutes started with induction and Group B received Magnesium sulphate (50 mg/kg in 30 ml of isotonic 0.9% normal saline intra peritoneally (IP) at the end of surgery. Results: The pain scores of IP group (intra-peritoneal group) were significantly lower than IV group. The total opioid consumption postoperatively in IP group was significantly lower than IV group. Also, there was a significant reduction in opioid-related side effects such as postoperative nausea and vomiting. Conclusion: Instillation of 50 mg of magnesium sulphate intravenously after completion of laparoscopic surgery renders patientsof less pain at the first 24 hours after surgery, more pain free period and less consumption of analgesics in the post-operative period.