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Rotary compressor used for refrigerant compression is the core equipment for room air conditioners(RACs). For an energy efficient RACs, energy efficiency metrics is moving to seasonal performance evaluation. And the compressor efficiency at low load conditions is getting more important than high load conditions. Therefore, compression efficiency under low load conditions is a major indicator of overall operational efficiency. This paper presents a design optimization procedure to improve the efficiency of a rotary compressor under low load condition using response surface method(RSM) and fluid-structure interaction(FSI) technique. Three shape dimensions associated with the discharge valve system were selected as design parameters, fifteen design points were planned using Box-Behnken scheme. The performance of each design point was evaluated by numerical analysis using FSI for the composition of response surface. In order to ensure the reliability, FSI techniques used in the performance evaluation were established through verification with PV test data. The optimum model for the valve system was derived by analyzing the constructed response surface. The responsibility of the optimum model was improved because it had relatively low stiffness compared to the base model. As a result, compression efficiency increased as the energy loss caused by the unstable behavior of the base model was reduced.