Abstract: There is mixing and second flow in sub-channels of the rod bundle. The flow and heat transfer characteristics in rod bundle are more complex than those in rectangular channel and round pipes. Nuclear power ships and floating nuclear power plants are constantly affected by sea waves and hence are under rolling conditions. Literature review shows that the rod bundle channel experiments are mainly under low pressure, while there are only a few studies on the high pressure flow heat transfer characteristics in rod bundle channels under natural circulation conditions. This research conducted many experiments and studied the flow heat transfer characteristics in the rod bundle channel under rolling motion. The main system included test section, condenser, pressurizer, controlling valves, flow meter, pre-heater et al. Natural circulation can be generated in the experimental system through the arrangement of the location of hot source and cold source. The test section is a 3×3 rod bundle channel and the heating length of the rod bundle channel is 0.8 m. Two thermocouples were installed on each cross section in each channel to measure wall temperatures and the pressure drop between each point was measured. The effects of rolling amplitude and rolling period on the rod bundle wall temperatures and heat transfer coefficient were obtained for subcooled boiling and saturated boiling. The rod bundle channel heat transfer coefficient relations were calculated. The experimental results show that during the rolling process, the system flow rate will fluctuate and result in the periodical fluctuation of the subcooled boiling void fraction distribution and fluid temperature fluctuation, and then lead to the periodical change of the heat transfer coefficients of the rod bundle. As the rolling amplitude increase, the fluctuation of the heat transfer coefficient also increases. The rolling period has a small effect on the subcooled boiling heat transfer coefficient. As the rolling period increases, the influence of the rolling motion on the flow in the rod bundle is weakened, and the fluctuation of the heat transfer coefficient is smaller. The heat transfer coefficients for saturated boiling fluctuate more strongly than the coefficients for subcooled boiling. The results show that for the experimental conditions, as rolling amplitude increases, the saturated boiling rod bundle channel heat transfer coefficients will increase. The average heat transfer coefficient during rolling motion is almost the same with the heat transfer coefficient at steady conditions for both saturated boiling and subcooled boiling, which means rolling has relatively small effect on the overall heat transfer despite of the periodical change of the transient heat transfer coefficient during the rolling process.