High-strength and high-toughness nanofibers were made from polyimide 6F-PI through electrospinning. The 6F-PI had a backbone made up with 3,3′,4, 4′-biphenyl-tetracarboxylic dianhydride and 2,2-bis[4-(4-aminophenoxy)phenyl]-hexafluoro-propane residues. Electrospun 6F-PI precursor nanofibers were collected in the form of aligned fiber sheet on the rim of a rotating disc. Heating process converted the precursor fiber sheets to 6F-PI nanofiber sheets. Gel permeation chromatography and Ostwald Viscometer were used to determine the molecular weight and the molecular weight distribution of the 6F-PI precursor, i.e., the 6F-polyamic acid. Scanning electron microscopy, infrared spectroscopy, X-ray scattering, tensile testing, dynamic mechanical analysis, thermogravimetric analysis, and differential scanning calorimetry were employed to characterize the surface morphology, thermal stability, and mechanical properties of the 6F-PI nanofiber sheets. Experimental results show that the nanofibers were well aligned in the sheets with fiber diameters ranging from 50 to 300 nm. The nanofiber sheets were stable to over 450°C, with a glass transition at 265.2°C. The uniaxial tension test showed that the 6F-PI nanofiber sheets had superior mechanical properties. The ultimate tensile strength, modulus, toughness, and elongation to break of the 6F-PI nanofiber sheets are respectively, 308 ± 14 MPa, 2.08 ± 0.25 GPa, 365 ± 20 MPa, and 202 ± 7%. It is expected that electrospun PI nanofibers with such high toughness and high ultimate tensile strength can find applications in high-performance textiles and composites, for example.