An EXPERT CAD SYSTEM for decision making in the design of a rotary shaft of the tiller has been built in this study by summarizing all the necessary results of our researches on the design theories, so that expert knowledge can be usefully applied to the planning design of the rotary shaft and the designer can also demonstrate his ability in the design process. As the results using the EXPERT CAD SYSTEM in the practical design, it enables the designer to achieve the planning design of the optimum rotary shaft quickly by means of conversation with the computer and inputting his desirable performance parameters of the rotary tiller into the system. In this paper, the authors report the following contents about the EXPERT CAD SYSTEM. 1) the building method and procedure; 2) the construction and components; 3) the whole flow scheme and operation method; 4) the final results of the deduction inference.