The purpose of this thesis addresses a development of an expert system to support a decision making of the vender selection. The researches related to the vender selection problems have been studied and they provide 23 criteria to select proper venders. In this thesis, 8 criteria have been used to construct a knowledge base of the expert system. The system in this thesis consists of 6 steps in its procedure. Step 1 decides a specification that satisfies customer's needs and Step 2 chooses a part supplied by a vender. The type of an outside order is decided in Step 3 and some venders satisfying the customer's needs are selected in Step 4. Some of the venders chosen from step 4 which do not satisfy the fatal cirteria(that is Quality, Delivery, Price) can be deleted in Step 5. In the last step, 8 cirteria is used to select 3 venders according to their ranking. Consequently, this program provides for a man, who does not have the experiances, an efficient way to select appropriate venders in the vender selection problems.