A series of AlN/Si3N4 multilayers with different Si3N4 thickness were synthesized by reactive magnetic sputtering. The microsructure of the multilayers was characterized with Xray diffraction and highresolution transmission electron microscopy, and nanoindentation was employed to measure their mechanical properties. The crystallization behavior of Si3N4 modulation layer in the multilayers and its influences on the microstructure and mechanical properties of AlN/Si3N4 multilayers were studied. The results show that when Si3N4 thickness is less than about 1 nm, Si3N4, normally amorphous in deposition state, could form a wurtzitetype pseduocrystal structure, same as the structure of hAlN, due to the template effect of AlN crystal layer. Crystallized Si3N4 layers and AlN template layers grow coherently into columnar crystals. Correspondingly, the hardness of the films is enhanced, showing a superhardness effect. Further increasing the thickness of Si3N4 layers, the coherent interfaces of the multilayers are damaged and Si3N4 layers become amorphous, accompanied by the decline in hardness of the films. The discussion indicates that the superhardness effect in AlN/Si3N4 nanomultilayers is related to the enhancement of modulus difference between the two different module layers caused by the alternating stress field in the coherent growth structure.