It is recognized that guaranteeing the interoperability of defense complex systems(SoS: System of Systems, FoS: Family of Systems) provides an information advantage, which is one of the essential factors for determining an warfighting advantage. This study presents the defense KIP(Key Interface Profile) process for improving interoperability of defense complex systems. The KIP process is to develop key interface profiles based on the key interfaces which are identified through the analysis of architecture. The KIP developed can be leveraged to manage the interfaces of complex systems. We verify the effectiveness of the proposed KIP process through a case study based on the systems of counter-fire operation.