Abstract: Crystalline and amorphous SiO₂/Al₂O₃ nanocomposite powders were prepared by a sol-gel method. The structure and morphology of the nanocomposite powders were characterized by X-ray diffraction and transmission electron microscopy, while the in-frared stealth property was detected by Fourier transform infrared spectroscopy spectrometry. The results show that both the nanocom-posite powders have irregular shapes, in which the average grain size of the crystalline one is about 18 nm. The crystalline nanocom-posite powder is a kind of composite nanomaterial with low infrared emissivity, and its average emissivity is 45.65% in 3-5μm and 46.19% in 8-14μm, which are lower than those of the amorphous one in 2.5-25μm.