Melatonin has been suggested as a new natural pain killer in inflammatory pain and during surgical procedures. We designed this randomized double-blind controlled study to evaluate the analgesic efficacy and also optimal preemptive dose of melatonin in patients undergoing cesarean section under spinal anesthesia.One hundred twenty patients scheduled for cesarean section under spinal anesthesia were randomly allocated to one of three groups of 40 each to receive melatonin 3 milligram (mg) (group M3), melatonin 6 mg (group M6) or placebo (group P) sublingually 20 minutes before the spinal anesthesia. The time to first analgesic request, analgesic requirement in the first 24hours after surgery, hemodynamic variables, anxiety scores and the incidence of adverse events were recorded.The duration of anesthesia and analgesia didn’t show significant differences between three groups. Total analgesic request during 24 hours after surgery was different among the three groups (P=0.035).The incidence of headache in group M6was significantly higher than others(P<0.001) .However, after adjusting headache between groups of the study, we were unable to show the significant difference in the total analgesic request during 24 hours after surgery among the three groups (p=0.058).Although premedication of patients with 3mg sublingual melatonin prolonged time to first analgesic request after cesarean delivery compared to placebo group, the difference was not statistically significant. Meanwhile increasing dose of melatonin to 6mg failed to enhance analgesia and also increase the incidence of headache in patients undergoing cesarean section under spinal anesthesia.