Inclusion Criteria:  Term primipara or multipara  Exclusion Criteria:  Severe pregnancy-induced complication (such as preeclampsia, eclampsia, poorly control pregnancy-induced hypertension and/or diabetes) High risk for postpartum hemorrhage Contraindications for neuraxial block Preterm (gestational age< 36 week) delivery Emergency cesarean section After-office hour schedule History of substance abuse Known allergy to nalbuphine, benzyl benzoate or sesame oil Not willing to follow the assignment of treatment after randomization