In this paper, a series of in situ nanofabrication techniques of nanostructures, including cutting, bending and welding of carbon nanotubes (CNTs), inside a field emission scanning electron microscope (FE-SEM) is reported. In the CNT cutting technique, a CNT was exposed to a low energy beam assisted with 1 seem oxygen gas flow. The proposed technique is effective being capable of cutting a CNT in less than 1 minute. It was found that, although the total pressure of specimen chamber reached 10-2Pa, high speed cutting occurs only in the area close to the nozzle. The presence of oxygen gas in the vicinity of the CNT can be applied also for the bending of CNT, if some conditions of the cutting technique are changed. These include the increase of the acceleration voltage and/or setting the oxygen gas nozzle farther from the sample, and/or reducing the irradiation time. Using the proposed bending method the angles larger than 90deg can be formed and the location of the kink can be set accurately. It is also shown, that tungsten can be deposited on a substrate by the electron-beam-induced deposition (EBID), if the oxygen of the proposed cutting technique is replaced by W(CO)6. In this paper, these three nanofabrication methods were exploited in creation of a two dimensional (2D) nanostructure, the letters N and U, and a three dimensional (3D) nanostructure, the letter N. The 2D letters were constructed from 6 CNTs assembled on a substrate while the 3D letter N was bended from a single CNT and fixed to stand on a substrate. Based on the high performance of the proposed techniques, it is suggested that the cutting, bending, and welding techniques inside SEM will become widely utilized in the fabrication and assembly of nanodevices and in the characterization of nanomaterials.