Shock-recovery experiments have been performed on mullite polycrystals in the pressure range up to 65 GPa by using a two-stage light-gas gun. The recovered samples have been examined by using X-ray diffraction method and transmission electron microscope observation. In the samples shocked at 40 and 49 GPa, an amorphization of mullite occurs. Mullite nanocrystals with grain sizes less than 10nm are dispersed in the amorphous phase, indicating that the relatively large starting mullite crystals become nanocrystals accompanied with the amorphization. In contrast, at 65 GPa, the mullite decomposes into amorphous SiO2 and γ-Al2O3. Very fine γ-Al2O3 particles less than 10 nm are observed in the matrix of the amorphous SiO2, suggesting that a rapid disproportionation reaction of mullite have been induced during short shock event. The crystallization of γ-Al2O3 may occur during a pressure-release process.