In the present work, homogeneous and flexible mullite nanofibers were successfully prepared by electrospinning method through the conventional diphasic mullite sol-gel route for the first time. Alumina sol prepared using aluminum acetate (stabilized with boric acid) was mixed with colloidal silica to prepare the precursor mullite sol. Polyvinyl pyrrolidone (PVP) was applied as polymer additive. General rules for preparing desired as-spun nanofibers and fiber morphological evolution were discussed, and elemental analyses and mechanical properties of mullite nanofibers were studied. It was found that uniform as-spun nanofibers were obtained when the mass ratio of PVP solution to precursor sol was 1.5. Upon calcining, Al4B2O9 and Al18B4O33 phase formed instead of gamma-Al2O3 due to the introduction of boron. Average fiber diameter decreased unceasingly with the calcination temperatures increased from 800 to 1200 degrees C because of the densification related to the phase transformation and the presented amorphous SiO2. After calcining at 1000 degrees C, relatively smooth fiber surface was obtained owing to the self-repairing ability for surface defects. Mullite nanofibers with preferable morphology possessed Al/Si molar ratio of 2.98, homogeneous elemental distribution, elastic modulus of 25.18 +/- 1.29 GPa and outstanding flexibility.