Abstract: Objective To determine the effect of fentanyl on 50% effective concentration (EC₅₀) of ropivacaine for postoperative epidural analgesia following gynecological surgery.Methods Sixty-five patients (ASA class I to II) scheduled for elective gynecological surgery were randomly divided into two groups to receive 20 ml ropivacaine (group R, n=33) or ropivacaine combined fentanyl (group RF, n=32) for postoperative epidural analgesia. The concentration of ropivacaine was adjusted according to double-blinded up-down sequential allocation. The efficacy was assessed using visual analog scores (VAS) and EC₅₀ calculated using method of Dixon.Results The EC₅₀ of ropivacaine was 0.098% in group R and 0.069% in group RF, showing significant difference (P<0.01).Conclusion EC₅₀ of ropivacaine for postoperative epidural analgesia determined by up-down sequential allocation is 0.098%, which can be decreased by the use of fentanyl.