The properties of polymeric nanofibers are determined by their internal structure. Although electrospun nanofibers have been widely applied in many fields, their internal structure is still not extensively reported, especially for amorphous nanofibers, which cannot be analyzed by studying the morphology of the crystal lamellar such as the crystalline nanofibers. In this study, the internal structure of electrospun amorphous polycarbonate (PC) nanofibers is investigated. The phase contrast and transmission electron microscopy images show that PC nanofibers exhibit a cylinder-like structure composed of molecular chains that are highly oriented along the fiber axis. This interesting cylinder-like internal structure may be the result of evaporation-induced phase separation in the polymer solution jet and the high strain rate in the electrospinning process. The variation of mechanical properties of PC nanofibers agrees well with the varied internal structure of the nanofibers with different fiber diameters. Due to the high degree of molecular orientation, as-spun PC nanofibers exhibit superior elastic modulus (6.2 GPa) and strength (780 MPa). The cylinder-like structure provides an insight into the internal structure of an amorphous electrospun nanofiber, which helps optimize the mechanical performance of amorphous nanofibers and fiber-based devices.