We report on the usage of a simple microfabricated device that works in conjunction with a quantitative Nanoindenter within a scanning electron microscope (SEM) chamber, for the in situ quantitative tensile testing of individual catalytically grown pristine and nitrogen-doped multiwall carbon nanotubes (MWNTs). The two types of MWNTs were found to possess similar strengths but different load-bearing abilities owing to the differences in their wall structures. Also, stress versus strain curves and fracture surfaces showed that while the pristine MWNTs deform and fail in a brittle fashion, the nitrogen-doped MWNTs deform plastically to varying degrees prior to failure. High resolution transmission electron microscope (TEM) images of the nitrogen-doped MWNT fracture specimens showed the presence of regions of reduced cross-section areas and kinks in close proximity to the fracture surfaces. The presence of nitrogen atoms in the graphitic sheets was assumed to have led to the formation of kinks whose motion induced by straining could have resulted in the plastic deformation of the carbon nanotubes.