In the present study, heat transfer phenomenon inside a vertical down ward condensing flow is studied experimentally. Our primary concerns are the two-phase flow pattern, condensation heat transfer coefficient, pressure drop in the direction of flow, and the condensing length. Also, the pressure fluctuation inside the condenser tube was measured. Only the annular flow was observed in our experimental range (maximum inler vapor flow rate is 3.8 g/sec and the cooling water (15$^\circ$C) flow rate is within the range of 7.5 1/min$\sim$37.5 1/min). The condensing length was about 2.60$\sim$3.85 m with the inlet vapor flow rate of 2.3$\sim$3.8 g/sec. The pressure drop across the condensing region was 7.5 kPa when the condensing length was 3.85 m. The pressure drop and the condensing length were checked by using the experimental correlations by others under the assumption of two-phase annular flow. Predicted value deviates from the measured value up to 10\% when the cooling flow rate is high (more than 12.5 1/min) but the maximum deviation increases up to 50\% when the cooling water flow rate is low (less than 12.5 1/min). The thermal resistance in the cooling water side is much larger than that in the condensing side, and the condensing length is more influenced by the cooling water condition. Condensation induced instability was not observed in the present experiment.
