Abstract: The heat transfer in forced convective condensation of immiscible R-113 and water in a horizontal smooth tube was studied experimentally. Seven types of internal spiral fin (ISF) tubes were used for studying the mechanism of heat transfer augmentation both for smooth tube and ISF tubes. The experimental results indicated that water and liquid R-113 flow patterns during condensation controlled the heat transfer rate and the mechanism was very complex. However, the flow patterns can be generally considered to be one of three ideal types, i. e 1. channeling flow; 2.standing-drop flow; 3. film-drop flow. The channeling flow model which assumed that heat transfer resistance of water and R-113 were in parallel was verified by the experiments. The heat transfer coefficient of channeling flow was larger than that of others. ISF tubes affected the condensate flow patterns and increased the Nusselt number of . tube side condensation by 1,13 to 5.32 times of pure R-113 condensed in a smooth tube with only a little increase in pressure drop.