Microstructure evolution study of TiAlN/CN x multilayer films with similar single layer thickness (the thickness of individual TiAlN layer: L TiAlN , L TiAlN /L CN x = 1) in the range of 0.9 nm to 13.7 nm shows that the structure of the TiAlN/CN x -MFs changes with the change of L TiAlN . TiAlN/CN x -MFs with L TiAlN of 0.9 nm exhibit a nanocomposite structure with embedded nano-sized crystalline TiAlN particles in the matrix. Due to the template effect of the TiAlN layers when L TiAlN is 2.6 nm, the CN x layers will partially crystallize and grow coherently with the TiAlN layers. Analysis of the different relationships between the TiAlN/CN x interface reveals one type of orientation relationship between the fcc TiAlN and crystalline CN x layers: (111) TiAlN‖(111) C 3 N 4 . Interfaces of the TiAlN/CN x are sharp but with the expansion of the TiAIN lattice spacing and the shrinking of CN X relative to itself, the lattice spacing will vary near the interface region. Further increasing L TiAlN , the CN x layers will exhibit completely amorphous structures and slowly lose their coherent interfaces, triggering a rapid decline in hardness and toughness. Maximum hardness of the MFs is 27.2 GPa when the L TiAlN is 0.9 nm, while the fracture toughness of the MFs with L TiAlN of 2.6 nm is almost 1.5 times that of the other MFs. To obtain high hardness and enhanced toughness in MFs, crystallization of CN x layers and coherent growth are required. • TiAlN/CNx-MFs with LTiAlN of 0.9 nm exhibit nanocomposite structure • Extremely fine TiAlN crystals were formed • The atomic mixing or the bond energy lead to the change of lattice spacing • TiAlN nc grains are separated by a matrix phase