In order to investigate the interfacial structure between TiN and iron nitride, an AISI 4140 steel was nitrided to form a layer of thickness 15 mu m by DC ion nitriding, then the surface was bombarded with Ti ions and subsequently coated a TiN film of 5 mu m by are ion plating method. The interfacial microstructure between TiN and iron nitride was characterized by optical microscope, SEM and XRD. The so-called 'black' layer was observed in the duplex treatment. It resulted from the decomposition of iron nitride during the bombardment. Its thickness was increased with increasing bombardment time at high bias voltage. But the thickness was greatly decreased when the iron nitride was bombarded with a nitrogen gas or at a reduced bias voltage. The adhesion strength of the top TiN coating was decreased with increasing thickness of the black layer. Furthermore, the reduced adhesion strength in this system was discussed in view of the interfacial structural relationship between TiN and iron nitride.