A series of AlN/SiO2 nano-multilayers with different SiO2 thickness were prepared by reactively magnetic sputtering. The microstructure of the nano-multilayers was characterized with X-ray diffraction and high-resolution transmission electron microscopy, and a nanoindentor was used to measure their mechanical properties. The phenomenon of crystallization of SiO2 in the multilayers and its effects on multilayer growth and mechanical properties were studied. Results show that SiO2, normally amorphous under sputtering condition, was crystallized when thickness was below 0.6 nm due to the effect of hcp crystal structure of AlN. And SiO2 formed a pseudocrystal structure just the same as AlN and grew epitaxially with AlN on the crystal plane of (0001). Because of the lattice mismatch between two layers, there existed an alternate stress field in the multilayer, which counted for a large portion of the hardness enhancement of the multilayer. Further increasing SiO2 layers thickness, the coherent interfaces of the multilayers were damaged and SiO2 become amorphous, accompanying by the decline in the hardness of the films.