Abstract: Mark XIIA is an new type identification foe or friend (IFF) system developed by NATO armies, and Mode 5 IFF is the core of the system. Firstly, the development and the framework of western Mark XIIA IFF system are summarized, the four operating modes of Mode 5 IFF system are introduced. It is shown that Mode 5 has not only the ability of IFF, but also the ability of battlefield situation awareness and data transmission. Secondly, from the viewpoint of security, capacity and reliability separately, the characters of the data format of Mode 5 Level 1 and Level 2 are analyzed in detail, the encrypted data and decrypted data in interrogation and reply are given. Finally, the characters of Mode 5 IFF are summarized.