Abstract: Objective To study the efficacy of Azasetron in prevention of transdermal fentanyl induced nausea and vomiting in cancer patients. Methods One hudred and forty patients with the moderate-severe cancer-related pain were randomly divided into test group(70 patients) and control group(70 patients).Transdermal fentanyl and Azasetron were given to test group in the first week and transdermal fentanyl alone to control group.The side effects of transdermal fentanyl，including nausea and vomiting，were observed within at least 2 weeks. Results (1)The incidence and intensity of nausea and vomiting in the test group were significantly lower than those in the control group during the first week.As to the second week，there was no significant difference between two groups.(2)The patients in the control group had more severe nausea and vomiting in the first week than those in the following week with a significant difference.No significant difference was found in test group between the first and second week. Conclusion Azasetron could effectively prevent the nausea and vomiting caused by transdermal fentanyl in cancer patients.