Introduction & Objective: Considering the unfavorable mental and physical effects of post-operative pain and the necessity of having effective drugs with least side effects, which should also be accessible and easy to use, this study was carried out to compare parenteral pethidine and sublingual buprenorphine on post-operative pain management of colorectal surgeries. Materials & Methods: Patients undergoing elective colorectal surgeries at the surgery ward of Rasool Akram hospital were assigned randomly to two groups of sublingual buprenorphine (0.4 mg tablets) and parenteral pethidine (75mg doses) receivers. The severity of patients’ pain and its changes were evaluated by Visual Analogue Scale (VAS). Also the drugs’ side effects and patients’ satisfaction were assessed. Results: The severity of pain, which had no significant difference in the two groups in recovery room, in the next measures, were always less in the group of sublingual buprenorphine receivers than in the group receiving parenteral pethidine. The Repeated Measures ANOVA test showed that the intervention group had a significant effect on patients’ pain severity (P<0.001). The amount of side effects in the two groups had not significant difference (P>0.05). Patients’ satisfaction in the sublingual buprenorphine receiving group was more than the case of parenteral pethidine receiving group (P<0.001). Conclusions: It seems that sublingual buprenorphine is an effective and less harmful drug in managing patients’ post-operative pain and its effectiveness is more than parenteral pethidine. Thus, considering the easier method of its application, it is recommended to use sublingual buprenorphine for the post-operative pain management of colorectal surgeries.