Using tetraethoxysilane, aluminum nitrate and aluminum fluoride as raw materials, the precursor of mullite was prepared by sol-gel process. When the precursor sintered at 1200°C, mullite ceramic was obtained. Differential thermal analysis, X-ray powder diffraction and scanning electron microscope were used to characterize the dried mullite gel and ceramic blocks. The results suggest mullite is synthesized by solid-phase reaction mechanism. X-ray powder diffraction indicates mullite is the main crystals phase in the ceramic specimen. SEM micrograph shows the mullite grains in the shape of short rod with length of 20 um when sintered at 1200 °C for 2 h and the grains grown up to acicular with length of more than 50 um when the treating time under 1200 °C achieved to 4 h.