Electrospun carbon nanofibers, produced from polyacrylonitrile (PAN) nanofiber precursors, with their superior mechanical properties, are promising candidates for manufacturing advanced polymer composites. Here, we report a series of tensile tests performed in situ under scanning electron microscope (SEM)/transmission electron microscope (TEM) observation, which show that the modulus and strength of electrospun carbon nanofibers can be enhanced through a simple mechanical constraint during the carbonization step in the electrospinning process. The constrained carbon nanofibers of diameter less than 150 nm were nanomanipulated inside the SEM onto a specialized microelectromechanical systems (MEMS) based testing platform and subsequently tested in uniaxial tension until failure. It was identified that both the strength and modulus of the constrained carbon nanofibers with sub-150 nm diameters are on average higher compared to their unconstrained counterparts by similar to 22% and similar to 31% respectively. Also, by evaluating the internal graphitic order of the constrained carbon nanofibers using TEM-based diffraction methods, we identified that the mechanical constraint during carbonization results in a better degree of orientation in the graphitic crystallites along the fiber axis. Finally, we use Weibull statistics for the deconvolution of the effects of diameter and the mechanical constraint on the tensile properties of carbon nanofibers. The Weibull analysis also showed that the comparatively superior strength of CCNFs is primarily due to better alignment of crystallites with the fiber axis. (C) 2017 Elsevier Ltd. All rights reserved.