Background: Recently, improvements in drug administration technology have intensified interest in the treatment of postoperative pain. this has resulted in increased use of continuous intravenous infusion of opioid and epidural opioid as alternative to traditional intramuscular administration of opioid. The goal of this study, therefore, was to document the effects of pain control and side effects following continuous intravenous infusion of morphine or meperidine and intramuscular meperidine following cesarean section. Methods: The vital signs, pain score, oxygen saturation and side effects were compared in 150 patients receiving continuous intravenous infusion of morphine, 30 ${mu}g/kg/hr$ (n=50, group 1); continuous intravenous infusion of meperidine, 150 ${mu}g/kg/hr$ (n=50, group 2); or intramuscular meperidine, 50mg/every 6hrs (n=50, group 3). Results: VAS (Visual Analogue Scale) was significantly decreased after 30 minutes of administration in all three groups and was significantly lower at 1 hour, but higher at 6 hours in group 3 than two other groups. Severe desaturation episode, defined as $SpO_2$<90%, occurred in the group 3(0.2%). Moderate desaturation episodes, defined as $SpO_2$ 91~95%, occurred more in group 3 than in group 1 and 2 (17.4% vs. 10.4%, 8.2%). The incidence of side effects were similar among three groups. Conclusion: The continuous infusion of opioid was more effective and safe method of postoperative pain control than traditional intramuscular injection.