BACKGROUND: Diclofenac is a nonsteroidal anti-inflammatory drug widely used as adjuvants for postoperative pain management with opioid sparing effect. The effect of diclofenac on postoperative opioid analgesia of morphine and meperidine was evaluated in 180 women after cesarean section. METHODS: One hundred eighty parturients were randomly allocated to four groups and each group had 45 women. The parturients were given loading dose of morphine in M group and meperidine in D group using intravenous patient controlled analgesia (PCA) device for up to 48 hours when the parturients awoke and complained abdominal pain. The parturients received diclofenac 75 mg every 12 hours intramuscularly followed by loading dose of morphine in MV group and meperidine in DV group. We evaluated the postoperative opioid requirement, numerical rating pain score, delivery/demand ratio, patient's satisfaction and side effects including respiratory depression, itching, nausea, urinary retention and dizziness. RESULTS: Diclofenac decreased over 40% of morphine or meperidine requirement and also pain score at 1, 2, 3, 6, 12, 24 and 48 hours in the use of PCA morphine and at 6, 12 and 24 hours in the use of PCA meperidine. And the incidence of sedation and itching decreased in MV and DV group. CONCLUSION: We concluded that diclofenac as adjuvant of opioid for postoperative pain after cesarean section could decrease requirement of morphine and meperidine, increase pain relief and decrease sedation and itching.