Abstract: In order to investigate interface structure of composite films,several solid solution structures and elastic properties of a single atom in transition metal nitrides are studied with first principle method based on density functional theory.The calculation shows interesting results:(1) A silicon atom could not form a solid solution in TiN,ZrN,HfN,or TaN crystallites.It is the same for a germanium atom in TiN crystallite.(2) As distance between crystallites becomes large,a silicon atom in TiN,ZrN,HfN and TaN or a germanium atom in TiN may form interstitial or substitution interface structures.(3) A silicon atom in NbN or a boron atom in TiN could form interstitial or substitution interface structures.(4) Interface structures of a silicon atom in VN or a carbon atom in TiN is a substitution structure.Studies of mechanic properties indicate that modulus of elastic,bulk,shear and Young's of lower energy substitution and interstitial solid solution is less than that of transition metal nitrides.