This paper describes Computer Aided Expert System for Designing transmission. Design knowledge is classified into 2 groups - “CLASS FRAME” showing the concept relative with subject of study and “INSTANCE FRAME” displaying some examples together with knowledges in the form of diagram defined on frame as image data. These data were hierarchically sorted out for problem reduction and very effective and easy to understand knowledge representation was achieved. Besides that, experimental knowledge was processed by either probability theory based on parameter of certainty factor determining the quantity of specified information by using predicate logic or fuzzy inference using membership function both of which, in turn, made it possible to obtain reasonable and still yet optimum answer. Expert tool of personal computer can be employed for this system, and external program such as gear specification calculation program or gear stress calculation program can be executed by any user.