The synthesis and characterization of mullite composite nanocrystaline from high-silica kaolin sampled from Tianyang County of Guangxi Zhuang Autonomous Region is presented in this paper. Rough kaolin is firstly grounded and processed to remove impurities, iron and carbon before burned to produce composite oxides with mullite as major components. The composite oxides are further synthesized into composite mullite nanocrystaline by using hydrothermal crystallization process under low temperature and normal pressure. Tests on the composite nanocrystalline by TEM, TG-TDA, and X-ray diffraction methods show that the nanocrystalline range in size from 40 to 100 nm and are well textured. They also show significantly enhanced thermal stability at temperature as high as 1000oC.