A new method of synthesizing mullite nanocomposite from raw natural kaolin is presented in the paper. Raw kaolin is processed by oxidation-reduction fining method and then baked to produce fine kaolin powder. The fine kaolin powder is further synthesized into mullite nanocomposite by open-system hydrothermal crystallization method under normal pressure. With proper controls of NaOH mole ratio in the hydrothermal solution, crystallization time, and constant temperature, the average grain size of synthesized nanocomposite changed from 80nm to 100nm. The morphology of the nanocomposite could gradually change from platy to short prismatic, lamellar, and needle-like with increasing nucleation rate.