Abstract: The relationship is studied between system testability index and the system element testability indexes,and the factors are analysed,which have influence on determining testability indexes.Then,the fundamental requirements,mathematical models and working procedure for testability allocation are derived.The paper presents four testability allocation methods that are applied to different conditions.The first two methods are based on system' s complexity (failure rate) and importance (fault affection).The third one is named weight allocation method in which some factors are considered carefully.The last allocation method is specially for those systems which contain some old units (products).These testability allocation methods are useful in practical system and equipment development process.