Abstract: Testability is the essential parameter to evaluate and guarantee the functional quality of products. In the light of the DFA/M thoughts，the DFT (Design for testability) method is studied. In first，the testability of common mechanical products is described. And the concept of general feature is put forward. Then the product generalized feature model for evaluating testability is set up. The rule-based and frame-based knowledge representation methods respectively used to express the testability rules and information about apparatus and tools for testing and measuring are presented. By this way，the knowledge library supporting DFT expert system is made. Base on that，the DFT expert system prototype is developed on the Pro/E platform. At last，it is verified by product instances.

Key words: DFT, Knowledge representation, Product generalized feature model, Testability