A simple and reliable welding method was developed to weld carbon nanotubes with the power supply here. The carbon nanotubes were synthesized chemical vapor deposition method and Multi-walled carbon nanotubes was uesd here. Firstly, apply less than 5 V voltages between carbon nanotubes when they were in close proximity under direct view of optical microscope. Then, let carbon nanotube contact with each other and increase the external voltage to 7–8V until carbon nanotube was attached to the end of the other, the two carbon nanotube join into a carbon nanotube. Furthermore, some experiments were implemented to analyze the reliability, the images of the weld joint and the weld strength all indicted this method were reliable.