Abstract: The experimental results of flow characteristics of vertically inverted U-tube steam generator(UTSG) primary side fluid under single-phase steady state natural circulation condition, which were conducted on natural circulation test facility in Nuclear Power Institute of China (NPIC), were presented. Following phenomena were observed from experiment： 1) The pressure of UTSG inlet plenum is lower than that in exit plenum; 2) The inlet plenum temperature of UTSG is sharply lower than that in hot leg. By analysis, reversal flow in some U-tubes of UTSG under single-phase natural circulation condition can be deduced. Experiment result shows that the flow resistance coefficient of UTSG under natural circulation is obviously larger than that under forced circulation as the occurrence of flow reversal in some Utubes of UTSG under single-phase natural circulation condition.