Carbon nanotube (CNT) can be used as a building block of nanoelectronic mechanical systems (NEMS). The main problems involved in nanotube based applications are related to the fabrication and assembly of the CNT. In this paper, in situ nanofabrication methods, involving cutting, bending, and welding of multi-walled carbon nanotube (MWNT) inside a scanning electron microscope (SEM), are presented. These three nanofabrication techniques are further used in the assembly of a CNT based nanostructure. A three dimensional (3D) nanostructure, the letter N, was assembled 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.