Introduction and Aim: In this study, we aimed to evaluate the analgesic efficacy and side effect potential of transdermal fentanyl and intravenous morphine infusion in oncologic patients complaining from severe oral mucositis pain. Materials and Methods: This study included thirty oncologic patients who developed oral mucositis during or after radiotherapy or chemotherapy. Patients were divided into randomized 2 groups. Group I included intravenous morphine infusion (15mcg/kg/h), Group II included transdermal fentanyl equivalent to the dose of morphine. Whereas pain was evaluated with Visual Analog Scale (VAS), sedation was assessed by five-point sedation scale. Side effects were noted. Results: There was no demographic difference between the two groups (p>0.05). Adequate analgesia was achieved in both of the groups (VAS≤3) and no difference was found between the VAS values of the groups (p>0.05). There was no significant difference between the sedation scores of the groups (p>0.05). Side effects were similar and nausea, vomiting, and constipation were encountered most commonly. None of the cases exhibited respiratory depression, urinary retention, and pruritus and deep sedation. Conclusion: We concluded that intravenous morphine infusion andequivalent dose transdermal fentanyl achieved an effective analgesia in treatment of oral mucositis pain. However, transdermal fentanyl might be preferred for severe oral mucositis pain due to its following characteristic features; easy-to-apply, non-requirement of intravenous line, and establishment of long term analgesia by just a single application. Key Words: Cancer, oral mucositis, analgesia, morphine, transdermal fentanyl