A radio frequency tissue welding system has been developed for occlusion of vessels during surgery. The system is designed to replace commonly used mechanical clip and suture ligation techniques. Other energy based ligation techniques are limited to use on small structures (<EQ 2 mm) due to slow heating, unreliable sealing, and charring/sticking to the forceps. The system consists of forceps and an RF electrosurgery generator, both of which are specifically designed for optimal tissue sealing. The method combines optimal pressure delivery to the tissue and energy delivery consisting of a high heat cycle, a low heat cycle and a cooling cycle. The generator output is also voltage limited and delivers high current in order to remodel the collagen in approximately 5 seconds with no sticking or charring. The vessel sealing system was compared to other energy based ligation techniques including ultrasonic sealing and other bipolar systems. The pressure required to burst the vessel was used for comparison. Average burst pressures on 3 - 7 mm arteries were 126 +/- 154 mmHg, 607 +/- 314 mmHg, and 913 +/- 304 mmHg for ultrasonic, standard bipolar, and vessel sealing, respectively. Histologic evaluation showed vessel wall fusion and minimal thermal damage to adjacent tissues for the vessel sealing system.