Background: post operative pain is an essential problem. Epidural infusion of two different doses of bupivacaine in pain management of post orthopedic surgeries was studied. Methods: In this double blinded randomized clinical trial we studied two groups of 42 patients undergone knee surgery. Based on block randomization, patients received two different concentration of bupivacaine (0.1% or 0.125%) in combination with fentanyl (1.7µg/ml). The beginning infusion rate was 5ml/h. Pain scores were documented by a blinded researcher 6, 12, 18, 24 and 48 hours after completion of surgery according to VAS tool. Patient satisfaction, complications and treatment failure (when increased volume of epidural infusion or other analgesics were required) were evaluated. Results: In higher bupivacaine dose group, post operative pain scores were not significantly lower in different hours and during the follow up (both p values less than 0.001). After 48 hours, patients satisfaction were more in the higher bupivacaine group. There were no significant differences in complications. Treatment failure was more significant in lower bupivacaine dose (33.3% versus 11.9%, p=0.03). Conclusion: continuous epidural infusion of both bupivacaine and fentanyl provide acceptable post operative pain control in orthopedic patients. Although higher concentrations of bupivacaine during first hours are more effective, lower concentrations are as effective as the higher one, during subsequent hours.