Abstract: An expert system of model reduction techniques (ESOMRT) based on artificial neural networks is studied and implemented. This expert ststem is appropriate for both expert users and non-expert users. It can choose proper reduction method based on specific high order model of continuous-time or discrete-time control systems and reduction requirements. It can also evaluate the reduction quality in time domain and/or frequency domain. We have tried to use the knowledge representation of combining procedural method with artificial neural networks(ANN) method and realize semi-automatic knowledge acquisition by using the relearning mechanism of ANN. There are three kinds of working patterns in this system and friendly man-machine interactive interface, its intelligence level is comparatively high and it has a certain practical value. The expert system runs on IBM-PC/XT and 386 machines.