Inadequate postoperative pain management can lead to physical and psychological distress in patients as well as impact surgical wound healing and increase the risk of developing postoperative delirium and cardiopulmonary and thromboembolic events. Severe postoperative pain may also result in the development of chronic post-surgical pain (CPSP), which in turn can lead to prolonged use of opioids and increased health-care costs. A descriptive survey study in 60 postpartum women who received cesarean section suggested that the presence of postoperative pain significantly reduced the willingness of breastfeeding and infant care. The incidence of CPSP after cesarean delivery has been reported to vary from 1% to 18% up to 1 year after operation. Intrathecal morphine (ITM) injection is considered as the standard pain management strategy for post-cesarean pain. However, the overall analgesic effect of ITM is about 8-12h and it is associated with pruritus, nausea/vomiting, urinary retention, constipation, mental status change, and respiratory depression. Therefore, the development of a safe, conveniently operated, and long-lasting analgesic strategy, which serves as background pain control modality up to several days after cesarean section should provide clinically beneficial advantages in the management of acute postoperative pain and prevention of CPSP in postpartum women. Naldebain® is prodrug of nalbuphine, which was approved by the Taiwan FDA in 2017. Naldebain® is rapidly hydrolyzed by tissue of plasma esterase to release nalbuphine. The bioavailability of nalbuphine following intramuscular injection Naldebain® was 85.4%, and it took approximately 6 days for the complete release of Naldebain® into the blood circulation. Therefore, a single parenteral injection of Naldebain® could provide long lasting analgesic effect in several phase II trials. However, Naldebain® has not been tested in the pain control after cesarean section. Therefore, this PI-initiated prospective, randomized, open-label, non-inferiority trial aims to investigate the clinical efficacy of Naldebain® in management of acute postoperative pain in term parturient who receive elective cesarean section to provide analgesic effect that is not inferior to the standard ITM and prevent the development of CPSP.