In designing an air-conditioner as one of home appliances, analysis of the refrigeration cycle is one of important design processes. The performance analysis using a simulation program includes the following routine work and timeconsuming efforts: data input, evaluation of result, and modification of input data. The iteration is continued until the best solution which satisfies all the design requirments and design constraints is obtained. Because of the differences among designers' experience, one designer may arrive at a different solution from one by another. These problems are barriers in designing an efficient air-conditioner. An expert system can be a solution to overcome the barriers in that it can decide on the best solution, and can be a place for accumulating design knowledge. It can change the design process from designer-centered into system-centered procedure where the accumulation of design knowledge accelerates the change. This thesis explanes an implementation of a design expert system based on a design software, CEL, which is being used by the field engineers. An expert system shell, Nexpert is used for the implementaion.