Abstract In this study, the performance enhancement potential by series operation in seawater-source heat pump system, assumed installed in Gangneung city near the East Sea in Korea, was investigated. An annual heating load for a typical Korean-style apartment house, which has an effective area of about 85 m2, was modeled by using TRNSYS program. An ambient condition was assumed to be in accordance with the Korean standard meteorological data, while the seawater temperature was calculated from the regression equation based on the measurement. A heat pump performance at full-load was calculated from the regression equation, which involves refrigerant's evaporating and condensing temperatures, based on a commercial screw compressor performance map. A part-load performance was also considered. Simulation results show that an annual heating performance of a seawater-source heat pump system can be improved by 8% or more by series operation under the simulation conditions considered in the present study.