Background: Tonsillectomy is one of the most common procedures performed by otolaryngologists. Intraoperative bleeding is a significant problem which requires hemostasis and causes prolonged operative time. Several different techniques are used to perform this operation. The vessel sealing system (VSST) has been widely used in head and neck surgery because of its effectiveness and safety.Objective: To compare intraoperative blood loss, operation time, postoperative pain, and postoperative adverse effects (bleeding and others) between VSST and cold knife tonsillectomy (CKT).Materials and methods: This project was conducted at the Department of Otolaryngology, King Chulalongkorn Memorial Hospital. Twenty-three subjects, designed for tonsillectomy and qualified in eligible criteria, were enrolled consecutively. Interventions were randomized and performed using VSST on one side of tonsil and CKT on the other side. Measured outcomes for efficacy and adverse effects were intraoperative blood loss, operation, time, postoperative pain in Faces Pain Scale-Revised, postoperative bleeding, and other adverse effects by blinded assessors until postoperative day 14.Results: Efficacy of VSST was significantly better than CKT in intraoperative blood loss and operation time. Intraoperative blood loss by VSST, CKT, and their paired difference was 1.00, 25.00, and 20.00 mL (median) (p<0.01), respectively. Operation time by VSST, CKT, and their paired difference was 3.70±2.27, 8.52±4.79, and 4.83±4.60 minutes (Mean±SD), respectively. There was no significant difference in daily (day 0-14) postoperative pain between VSST and CKT groups. Only two cases of delayed (after 24 hours) postoperative bleeding were found on CKT side.Conclusion: VSST was better than CKT in efficacy (intraoperative blood loss and operation time) but not different in postoperative pain.