[en] Two-phase countercurrent flow in reactor hot leg at loss of coolant accident is studied by experiment and analysis. The test section consists of a vertical tube, an inclined tube and a horizontal tube. It is found that the length of horizontal tube has significant effects on the onset of flooding. The experimental research and theoretical analysis on the onset of flooding were done. Analysis and comparison of calculation model are given. The correlation for calculating the onset of flooding in reactor hot leg is recommended