Abstract Complex steam-liquid two phase flow exists in the heat transport pipeline connecting evaporator and condenser. Especially the flow condensation has important influence on the performance of the cooling system. The flow pattern and pressure drop in the heat transport pipeline were deeply studied in this paper. Firstly, electrical capacitance tomography was used to discern flow pattern in the heat transport pipeline. The flow pattern was compared against the predictions of Mandhane et al. flow pattern map and determined as stratified flow. Secondly, the pressure drop was measured and experimental results indicated that the pressure drop increased along with the increase of the heat load and was higher in long heat transport pipeline than in a short one. The increase in the pressure drop was also observed for smaller inclination angle and lower vapor quality. When the experimental results were compared with other five typical correlations in the literature, high deviations were observed. Then a new correlation which took into account the effects of tube diameter, length and inclination angle was developed. This correlation predicted the pressure drops within the range of ±30%. For the correlation, all droplets were regarded as a rough surface rather than a smooth surface in the tube according to the visualization results. So the friction coefficient was closer to the real experimental condition.