Abstract: An experiment on flow reversal in U-tubes of steam generator (SG) has been made within the range of flow rate 0.4~0.7 kg/s under natural circulation. Nine pairs of monitoring points corresponding to nine groups of U-tubes with different length were installed to obtain the distribution characteristics of flow reversal. Based on the conservation laws and test data, total flow rate and the number of U-tubes of reverse flow in the primary side of SG were obtained. The results show that the flow reversal occurs in about 61% of U-tubes, which reduce the flow area of heat transfer tubes to 39%. Because of the flow reversal, the flow rate of the forward flow rise by 60%, and the speed of the flow in U-tubes increase to 4.2 times in contrast with that without flow reversal.