Fly ash cenosphere/mullite (FACs/M) composites with excellent high-temperature resistance were prepared through a tert-butyl alcohol gelcasting process, using the mullite and FACs as the matrix and the filler, respectively, and the matrix of mullite was derived from phase transformation of the raw material of kaolin. Results show that the FACs/M exhibited low density and high specific compressive strength and the thermal conductivity was distributed over in the range of values predicted from Maxwell-Eucken model 2 and Effective Medium Theory (EMT) equation. Exponent m describing the relationship between compressive strength and porosity was 2.37. Young's moduli of the composites were lower than that predicted from the Hashin-Shtrikman model. The high-temperature testing results revealed that the FACs/M composite exhibited high compressive strength and the 50% FACs/M composite displayed compressive strength of 14.11 MPa after testing at 800 degrees C. The FACs/M composites with excellent mechanical properties both at room and high temperatures may possibly be used in the deep sea and other fields in the near future.