Currently, the strengthening mechanism and microstructural model of nanocomposite films are controversial. The aim of the present study is to elucidate the microstructure and strengthening mechanism of the NbSiN nanocomposite film. A series of NbSiN films with different Si contents are synthesized by reactive magnetron sputtering, and the films are characterized as nanocomposite structures with NbN nanocrystallites surrounded by the Si3N4 interfacial phase. When the Si: Nb ratio is <1:4, the Si3N4 interface can be crystallized, and it prefers to grow coherently with the adjacent NbN crystallites, leading to the strengthening effect of the film. The maximum values for the hardness and elastic modulus of the NbSiN nanocomposite film reach 34.2 GPa and 287 GPa, respectively. Using the high-resolution transmission electron microscopy (HRTEM) observations, we verified that the Si3N4 interfaces can be found in a crystallized state when the NbSiN nanocomposite film is strengthened, and can coordinate the misorientations between the adjacent NbN nanocrystallites and preserve the coherently-epitaxial growth with these nanocrystallites. The strengthening effect of the NbSiN nanocomposite film can be explained by the coherent-interface mechanism. (C) 2017 Elsevier B.V. All rights reserved.