Objective: To evaluate the side effects and nausea and vomiting prevention of the patient controlled analgesia method (PCA) by intravenous fentanyl combined with ondansetron after thyroidectomy. Subject and method: 80 patients were performed thyroidectomy under cervical plexus anesthesia in the operating room of the No.103 Hospital from May to December 2013. These patients were divided into two groups: - Group 1 (the PCA group) (n=40): 100 ml PCA solution was prepared by dilution of 1000mcg fentanyl, 12mg ondansetron, and normal saline 0.9 percent. The concentrations of fentanyl and ondansetron in the PCA solution were 10mcg/ml and 0.12mg/ml respectively. After finishing thyroidectomy, when the visual analogue score (VAS) 4, titrating and setting up PCA as follows: Bolus dose 1 ml, lockout time 15min, background dose 1 ml/h and total 4 hour dose limit 20ml. PCA stopped after 48 hour of analgesia. - Group 2 (the control group) (n=40): After thyroidectomy when VAS 4, intravenous injection of ketogesic 15mg every six hours for 48 hour postoperation. Result: In the PCA group, the postoperative nausea and vomiting (PONV) rate was 22.5 percent, headache 5 percent and abdominal pain 5 percent, lower than those in the control group (60 percent, 22.5 percent and 15 percent, respectively) (p0.05). The rate of sedation OAA/S4 was 2.5 percent, dizziness 15 percent, pruritus 2.5 percent and urinary retention 12.5 percent, which were not statistically different from the control group (p0.05). Conclusion: Patient controlled analgesia method by intravenous fentanyl combine with ondansetron reduced significantly postoperative nausea and vomiting and side effects after thyroidectomy.