Abstract: Mullite fibers were prepared from aqueous solution of aluminum nitrate, aluminum isopropoxide and tetraethylorthosilicate by sol gel method. The formation of mullite fibers was studied by scanning electron microscope, TGA analysis, X ray diffraction, and IR spectra. MgO was introduced into the fibers by adding Mg(NO3)2. Mullite crystallization temperature decreased by 100 ℃ with the presence of MgO(wMgO=1%) and the main crystalline phase of the as prepared fiber was mullite as observed by XRD and IR. As shown by SEM, the fibers had a diameter about 20 μm and smooth surface without obvious defects with low amount of MgO, while overdose of MgO affected the surface morphology. Taken together, adding w=1% MgO could both reduce the mullite crystallization temperature and obtain mullite fibers with good surface at low temperature.