Graphitic carbon nanofibers (GCNFs) with diameters of approximately 300 nm were developed using bundles of aligned electrospun polyacrylonitrile (PAN) nanofibers containing phosphoric acid (PA) as the innovative precursors through thermal treatments of stabilization, carbonization, and graphitization. The morphological, structural, and mechanical properties of GCNFs were systematically characterized and/or evaluated. The GCNFs made from the electrospun PAN precursor nanofibers containing 1.5 wt.% of PA exhibited mechanical strength that was 62.3% higher than that of the GCNFs made from the precursor nanofibers without PA. The molecules of PA in the electrospun PAN precursor nanofibers initiated the cyclization and induced the aromatization during stabilization, as indicated by the FT-IR and TGA results. The stabilized PAN nanofibers possessed regularly oriented ladder structures, which facilitated the further formation of ordered graphitic structures in GCNFs during carbonization and graphitization, as indicated by the TEM, XRD, and Raman results.