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Abstract. Transient heat transfer of a condenser and its effect on a cascade heat pump performance are investigated by comparing experimental results with the prediction from a mathematical model. In the experiment part, the cascade heat pump consists of the lowtemperature refrigeration cycle using R22 and the high-temperature heat pump cycle using R134a. The transient effect is presented by circulating water to receive the rejection heat from the condensing refrigerant of the cascade heat pump causing the rise of the hot water temperature. On the other hand, a mathematic model of the condensation heat consisting of the heat transfer models in the two-phase mixture and the superheated vapor regions is developed. The effect of the hot water flow rate on the hot water temperature and the condensation heat is examined. The result indicates that the hot water temperature continuously increases with time. The condensation heat rapidly increases at the beginning and gradually increases with time. The agreement between the results obtained from the experiment and the model prediction is obtained. The result from the model prediction indicates that the heat transfer on the R134a side dominates the total heat transfer mechanism. The experimental result shows that the COP of the heat pump increases with increasing hot water flow rate, leading to the higher efficiency of the heat pump.