In order to study the superhardness effect of superlattice films, a series of TiN/NbN superlattice films with various modulation periods were synthesized by reactive sputter deposition. X-ray diffraction analysis, transmission electron microscopy, and microhardness analysis were employed to characterize the modulation structure, interface structure and microhardness of these superlattice films. The results show that TiN/NbN films possess good periodic modulation structure and the modulation interfaces are straight and clear. The superlattice films have a face-centered-cubic polycrystalline structure resulting from epitaxial growth. They assume unusual microhardness which can reach a peak value of HK 39.0 GPa at a modulation period of 8.3 nm. It is considered by analysis that the superhardness effect of TiN/NbN superlattice films results from the strengthening effect of an alternating stress field, which is caused by the epitaxial growth of two kinds of materials with different lattice constants.