A computer aided system has been developed as an aid to the preliminary selection of agitators for stirred vessels. This paper is concerned with modeling of the agitator selection process for a particular application and for general applications using the analytic hierarchy process (AHP). It is concluded that the decision-making results are fairly consistent with the human experts decisions. The knowledge manipulation of the system has been presented from three aspects: knowledge acquisition, knowledge representation, and the methodology of handling uncertain data using the theory of fuzzy sets. The implementation of the system is illustrated by the operating procedures.