Fine-grained mullite nanofibers derived from the diphasic mullite sol were successfully fabricated by electrospinning and subsequent pyrolysis at 1500 degrees C. Polymethylsiloxane and aluminum tri-sec-butoxide were selected as the silicon and aluminum source to synthesize the diphasic sol. Results show that the weight loss of mullite precursor fibers in our work was about 60 wt.%, which is similar with that of fibers fabricated using the monophasic sol. This low weight loss was mainly attributed to the high ceramic yield of polymethylsiloxane and low introduced polyvinylpyrrolidone content, which ensures the integrity of fiber morphology during the sintering process. Mullite fibers with 216 nm average diameter were fabricated after sintered at 1500 degrees C and the corresponding grain size was only similar to 100 nm, much smaller than that in mullite fibers derived from monophasic sols. Therefore, it can be predicated that mullite fibers in this work should possess a higher mechanical strength than those derived from monophasic sols when the sintering temperature was higher than 1400 degrees C and therefore was an ideal starting materials for the fabrication of mullite nanofibrous ceramics used as the high-temperature thermal insulation materials.