The xerogel of mullite precursor with high specific surface area (422m2•g-1) was successfully prepared from the sol of aluminium isopropanol (AIP) and tetraethyl orthosilicate (TEOS) tuned by supercritical CO2 and subsequent supercritical fluid extraction. Nanometer mullite was prepared by calcining the xerogel of the precursor, and the obtained mullite together with the xerogel precursor was characterized by SEM, XRD, N2-adsorption and IR. The results showed that the xerogel precursor exhibit a shape of amorphous structure, with size of no more than 100 nm, and no huge blocks was found, indicating that the supercritical CO2 plays an important role in controlling the process of hydrolyzation, sol-gel transition, in which supercritical CO2 may promote the mixture of Si and Al on atomic scale. SEM indicated that the obtained mullite were mainly 2-3 μm in length and lath in shape with aspect ratios of 6-10, and the laths were interlaced with each other to construct a relative dense mullite aggregation. The mineralogical phases tracing experiment showed that the mullitization process underwent from 1200 °C to 1400 °C as followed by the phase transition from -Al2O3 to -Al2O3, and to -Al2O3, and the temperature of complete phase transformation into mullite was 1400 °C.