We have fabricated n-type carbon nanotube field effect transistors by choosing the contact metal. Single-walled carbon nanotubes were grown directly on a SiO2/Si substrate by chemical vapor deposition using patterned metal catalysts. Following the nanotube growth, Ca contacts with a small work function were formed by evaporating and lifting off the metal. The devices showed n-type transfer characteristics without any doping into the nanotube channel. In contrast, the devices with Pd contacts showed p-type conduction. These results can be explained by taking into account the work functions of the contact metals.